They measure difficulty pre-release by looking at stats (first) and getting feedback from testers (distant second). If they see stats that show a difficulty spike they didn't expect in an event, they ask the testers what's happening. If it's more difficult than they want it to be for that piece of the game, they change it. This is the first mission of a new game that they're trying to sell to as many people as possible. They want it to be a smooth experience that draws players in. Get as sweaty as you want about this, but they're making business decisions at the same time they're making game design decisions, whereas we are sitting here jawjacking with no skin in the game. It's easy to demand changes when you risk nothing on the outcome.