Correct. It's not a flat boost. It's a bell curve based on target and difference in roll. As it's a bell curve, it's just wrong to describe it in terms of its maximal value. Of course you're going to have those lucky "1 turned to 20" rolls as well but they are exceedingly rare. I'm speaking in terms that represent the whole array of values because that's just the honest thing to do when you're talking about a curved outcome.
The flat boost option is there for people who really don't want to trust the die. For some, it's better to roll an 8 and being able to "reliably" get a 13 vs. rolling an 8 with adv. and the second roll be lower. The probabilistic result is still random any time you roll a die. So sure, you can say you have a coin toss's chance of increasing your base odds but it can just as frequently not.