I think you need to realize that the Democratic Primaries where only Democrats vote and the general election where everyone can vote are two different things. They involve completely two different sets or types of people. Well, democrats can vote in both, but Republicans can't vote in the democratic primaries and most independents can't vote in the democratic primaries either unless they live in an open primary state.
As for Super Tuesday, it's the same thing. Only democrats are voting in their primaries, not Republicans nor independents for the most part. Sanders is at this point in time, just winning among democrats, not the American public as a whole. Just among one of three major factions. Say just a third of the total electorate.
Look at this way, Hillary Clinton was the choice of democrats in their primaries, she lost when all Americans could vote. Obama was the choice of the democrats in their primaries, he won when all Americans could vote. Gore and Kerry were the choice of democrats in their primaries, both lost when everyone could vote, etc. Being the choice of and receiving the most votes in the democratic primaries doesn't mean they will be the choice of all the people in November.
Following what the pundits state, me personally. I know I won't vote for Trump in November of 2020. Now that doesn't make me an automatic vote for the democrats either. That depends on who they nominate. I'd vote for Biden, Klobuchar, Gabbard against Trump. I would vote third party again if the democrats nominate Sanders or Warren like I voted against both Trump and Clinton in 2016. The rest, Bloomberg, Buttigieg, I haven't made up my mind. What this means is candidates matter to me. Not party, but candidates.
The primaries are one thing with one type of voter, just Democrats. The general election involves every type of voter, not only democrats, but everyone. There's your difference.