I earned the "So Hot Right Now" achievement this morning, and the "Uldaman: Legacy of Tyr" achievement this evening, so one would expect the Uldaman achievement to appear at the top as it's the newest achievement.

I've noticed this happening many times, but didn't bother to screenshot it and report it until now.
I assume it's because it's sorting by date instead of epoch time? Or maybe I'm just guessing blindly.

The only data blizz stores is year, month, day. There is no further detail so I assume it defaults to the name when the date is the same. Right now there is no plan to add hour, minutes and seconds since this would be lost in case your WTF folder gets wiped.