Re: DnD 5e Xp calculation
I don't know if this still applies to 5e, but part of CR in 3.x was not just how difficult a creature was but also based on the average expected numbers such as saves, DCs, etc as well as whether players were expected to be able to bypass certain defenses. So just because players were dealing lots of damage and killing creatures off quickly didn't mean that simply raising the CR was the answer, because raising the CR could completely unbalance other factors such as the chance for the caster to affect the monster, the chances of whether the players could resist the monster's abilities etc.
Thus the CR system broke down, because the players could overspecialize in one area to totally crush enemies while their other stats were still normal, and any monster that actually used that against the players would seem like it was too powerful when the ability was basically not resistible, yet the monster would still die too easy.
Not sure I explained it very well, but I hope you get what I mean.
More of the same CR is a different kind of difficulty increase then using a higher CR.