How much do you understand about law? Its all about interpretation. Look at the 14th Amendment for example, specifically the equal protection clause in section 1. It states:
All persons born or naturalized in the United States, and subject to the jurisdiction thereof, are citizens of the United States and of the State wherein they reside. No State shall make or enforce any law which shall abridge the privileges or immunities of citizens of the United States; nor shall any State deprive any person of life, liberty, or property, without due process of law; nor deny to any person within its jurisdiction the equal protection of the laws.
That last phrase, equal protection of the laws has changed meanings since its creation and addition to the Constitution. For example one could make an argument that women should have been giving the right to vote under that clause, as all persons are granted equal protection. Yet we needed the 19th amendment to grant women the right to vote. Today there are cases working there way up to the Supreme Court and other high courts which state that laws prohibiting gay marriages are unconstitutional under equal protection, and one could make an argument for that. Quite clearly the homosexual community sees laws which stand in the face of their pursuit of happiness and their opinion of what equal protection is, that is based on the fact that in most states they cannot marry but others can, they see that as an inequality.
Whether thats a correct interpretation I can't say but what I can tell you is just like in the past where segregation was once Constitutional under the 14th Amendment and now is not, interpretation defines Constitutional law. Hell just recently the Court overturned a campaign finance law under a NEW interpretation of the 1st Amendment, that wasnt the first time that law was brought before the Court but it was upheld one time and denied another time. Why? The 1st amendment didnt change, but the interpretation did.
Even your all encompassing view that what the Constitution says applies equally and samely everywhere is wrong, it NEVER has. Gun laws, religious laws, alcohol consumption and sales laws, have always been different for each state and county. Never in all of US history have the same laws which regulate items addressed in the Constitution been the same throughout all the states. Massachusetts had an official state religion for decades where individuals were required to pay a special tax if they were not members of the Anglican church, it was considered fair partly because church members would be paying a tithe. You think that would fly today in any court? Hell no, but the first amendment hasnt changed. And why is that? Because people want to live differently.
Lastly, look at Article 3 of the Constitution which defines the role of the Supreme Court.
The judicial Power shall extend to all Cases, in Law and Equity, arising under this Constitution, the Laws of the United States, and Treaties made, or which shall be made, under their Authority; to all Cases affecting Ambassadors, other public Ministers and Consuls; to all Cases of admiralty and maritime Jurisdiction; to Controversies to which the United States shall be a Party; to Controversies between two or more States; between a State and Citizens of another State; between Citizens of different States; between Citizens of the same State claiming Lands under Grants of different States, and between a State, or the Citizens thereof, and foreign States, Citizens or Subjects.
The Constitution itself vests the power of its own interpretation to the Supreme Court, in my opinion. This interpretation was also reached in Marbury v. Madison in 1803, establishing Judicial review, and hasn't changed in the 200 years since. So under the Constitution the Supreme Court decides what the Constitution means, which means the current gun laws for example as they exist in certain states are Constitutional until the Supreme Court says they are not. And if the Supreme Court thought there was a need to redefine or re-enforce its viewpoint on the Constitution it would take a case relating specifically to one of the Amendments of of Constitution. For example in 2008 they felt this need to re-enforce its viewpoint of the 2nd Amendment in District of Columbia vs. Heller, in which they decided with a 5-4 vote that, among other things,
1) Handguns were defined as arms, which had NOT been defined by the Supreme Court until now.
2) DC's law banning handguns was therefore unconstitutional.
3) Reaffirmed the right to own a firearm in all federal enclaves.
4) Owing a firearm without connection to a militia was Constitutional
5) Declared parts of the Firearms Act of 1975 unconstitutional
It also Held that: The Second Amendment guarantees an individual's right to possess a firearm unconnected with service in a militia, and to use that arm for traditionally lawful purposes, such as self-defense within the home. United States Court of Appeals for the District of Columbia Circuit affirmed.
You'll notice they didn't declare a strict literal interpretation of the 2nd Amendment that you advocate when they had all opportunity to do so after taking a 2nd Amendment case. But under the Constitution their interpretation has reinterpretation the Constitution, even though it quite clearly does not follow the strict literal meaning. What you need to understand is that law is NOT about literal meaning its about legal meaning and about precedence. We might consider an arm to be all sorts of things that under its literal definition would clearly be an arm, but under its legal definition may not be. The Supreme Court here had to redefine an handgun as an arm in their own decision, if that doesn't show you the difference between legal and literal definition nothing will.
And as a final note, the purpose of the Court is NOT to match literal and legal definition, that isn't how law works again. For example the literal definitions of contract and agreement may be very similar and for all practical purposes in everyday language may have the same meaning, but legally they have separate and distinct meanings. If the two were merged to match literal definition it would destroy the meaning of the law.