I think Chris has the right of it, although to me it’s not bug, but a feature; tweaking encounters on the fly is part of the art of being a good gamemaster. In such an open game, it’s really easy for players to go totally off the rails and you can wind up throwing out half of your planned campaign let alone adjust a battle. However, you need to do this in order to keep your players engaged.
Of course, Tom is right; the developers are typically the gamemasters and if they’re pawning off this part of it to the player, it can kill immersion and therefore investment in playing. On the flipside, I’ve played Pathfinder/D&D/what not with tons of people over the years. A sizable majority had opinions on whether something was too hard or too easy and chimed in on how they’d prefer it, and that’s cool. However, most never took up the mantle of GM and so having this sitting out there may serve as a little bit of wish fulfillment.