[Bungleau]

I know Radek has explained it well... here's the basic summary.

Every time you level up, your HP are re-rolled for that class at that level. If the new roll is more than your current total, then you get the new roll as your total HP. If the new roll is less than or equal to your current HP, you get 1 HP.

So if you were a 20th level barbarian, with more HP per level, and then switched to a warlock, with fewer HP per level, you'd be gaining one point per level until you are way, way up there. You might get an occasional multi-point boost, but rarely.

And that's why we say it evens out, and it's not worth reloading to try levelling up again. It all works out in the end... as you're seeing now.