The thing is, in the same wiki article, it says "in the present day a number of earldoms take their names from towns, mountains, or simply surnames", and the http://www.uesp.net/wiki/Oblivion:Imbel_Genealogy seems to imply Jakben doesn't actually own any land, and his title is just taken from his surname, which is a little odd for the medieval setting of Oblivion, but what can you do.
I mean, he's a scumbag vampire, so you can't really trust that he's even an Earl at all, but I'm assuming the Elder Council (or someone high up anyway) would've noticed this discrepancy and had him executed for impersonating a noble, so I'd like to work on the assumption that he is actually a noble.
So, what's the verdict? County Imbel, real or not? :shrug: