Hmm... I probably just saw the post that created your confusion -- sounds like things might be a little up in the air... For now, I'd stick with the Disney published days for Extra Magic Hour. If you're going in the near future, you might just want to ask when you get there if they're switching anything around, both at your resort and then again at guest relations in the park.
It could be they just higher than normal attendance that one day so they extended the hours on an impromptu basis. If it happens again, I'm sure people will report here on the boards!
I would LOVE it if on-property guests got a few more perks for all the $$ we spend!