Opponents book not available - only FoG

[Najanaja]

On a couple of occasions recently I've accepted challenges that had no book specified (normally all books are available in this case).
When I've gone to select an army I've only had the FoG book available (ie Rome or Carthage only). But it's clear from my opponents army that he has chosen from a different book that wasn't available to me.
Anyone else seen this?
Robert