First of all, I said otherwise equivalent, not identical. 2d6+2 damage with a 75% chance to hit and 2d12+5 with a 50% chance to hit are equivalent (9 average damage per attack) but not identical.
Second, I pulled those number out of nowhere. They were an attempt to loosely illustrate the concept, but I have a better example that can show you the designers though processes. Look at the Skeleton vs the Zombie in the playtest bestiary. The Skeleton and the Zombie are both viable level 2 monsters, but the Skeleton is a bit more powerful and therefore worth 30 extra xp.
Sorry, I didn't mean to nitpick at your precise wording. I know that you were speaking in randomly chosen examples, I was just using simplified examples in response to make my point. I think it still stands. It's impossibly complicated to adjust xp values based on minor differences, and rather insignificant even if executed properly. I don't see it as being worth the effort.
My griping about monsters that are identical except for tiny differences is a different can of worms, like I said. I may have griped about that as an unnecessary aside.