Can you give any evidence indicating this at all?!
Uh, pay attention to the plot as it unfolds? :P

All of this stuff is mentioned in-game, in the manual or in the official player's guide.

So what? FFTA was out first. FFXII is based on FFTA, not the other way around.
I look at it this way:

Final Fantasy XII occured first
Final Fantasy Tactics occured next, after the great flood had taken its toll on the land and Humans were of the few survivors
Final Fantasy Tactics Advance occured last, in present day