I've received exploration XP for a few areas I've visited many times before, nothing huge, just odd. So far they include the Sis Liman desert area, the Barrier to the Forest, the Lairs tunnel north of the Conclave and the Pass of Rabbanatha.
I think none of these areas used to give exploration XP. I suspect they implemented them to give it from now on, hence you receiving it just now.
It also happens when the area has been changed to a new version and the module is updated.