The difference is that there are folks who like both chocolate and mint all the time. Thus they are not fluid - they know what they want and it does not change. Thus when others say things to them like "you might say that, but I know you really just like chocolate", or "sure you like chocolate and mint, but you obviously like chocolate more" or even "you're just saying that, because I haven't seen you have mint in like forever" can be insulting.
Basically, if someone identifies as X, the key is to be respectful of that (assuming X is ethical obviously) until they identify otherwise.