You left out the single most important factor. Power.
Police have an incredible amount of power. In the history of mankind, society has always placed a higher standard of behavior and honor on the holders of power. The Romans executed governors who erred, not the Centurians who did the deeds. Ships captains are expected to be the last off the ship, a pilot drinking on the job risks lives and therefore loses his career as opposed to a roadside suspension.
Police, good ones I have known, know and respect that. They know their testimony carries more weight with a jury, they know that they're mere arrival can alter the scene. They know that what they hear going into an incident is likely wrong, that just because someone may have thought they saw a gun, they wait to see one for themselves before opening fire.
The good ones.
What triggers me, if you will pardon the reference, is the usual knee jerk reaction from the gun lobby, wannabees and ex-military, is the immediate assumption the suspect is lying and that all cops are good.
They aren't. And when they're bad, the results are far more injurious and harmful than say a dentist who screws up a filling....
Thus higher standards.