Let's also not pretend that Sanders isn't capable of poli-flip-flop speaking, either. In July, he did an interview with Couric and said that pot was a “gateway drug” that can lead to heroin and cocaine use." Now, three months later, he's for decriminalizing it. (And I don't fault him for having a change of heart. Intelligent people listen to facts, reality and can often change their opinion. Unless it's economic policy out of the 1960s......)
He is a politician who is fortunate enough to live in a liberal state where his liberal positions don't require him to form deep coalitions with people who disagree with him, because, liberal Vermont typically does agree with him on most things. Interestingly, where he's been out of the mainstream on modern liberal thought (i.e. guns and immigration reform) it's because he claims he has to represent his state's (and in the case of immigration) unions interests. That's not a problem, but it shows that he's maybe not the fount of all that is liberal and pure. He's a good person, with some good ideas and some not so good ideas. Like when he voted to allow guns on Amtrak trains. Or voted against the Brady Bill the first, second, third, fourth and fifth time. Or when he worked to make sure nuclear waste didn't go to Vermont, but, instead, went to a poor, mostly Mexican-American area of rural Texas. So, there are times where he'll sell his liberal values for the benefit of his constituents. We just, to paraphrase the old joke, are simply haggling over the price.
Now that he's trying to build a coalition of supporters, he (and his supporters) are going to find that you need a bit of pragmatism and concessions to win an election. Instead of just accepting that, though, some (Read: some not all) Sanders supporters start doing mental gymnastics of why he's not really changing his mind, or why he's always felt that way because reasons. There are two Bernie Sanders, the man and the myth. One of them will survive the primary, the other won't.