Ooops, forgot to mention : in this game, I hadn't built any adept yet (got beaten by 3 turns to the catacomb library, and my toundra-isle towns wouldn't build a mage guild fast enough at the moment so I skipped magic for a while), but when I had a Ritualist learn Entropy II and begin summoning imps, I noticed that sometimes the imps could summon a skeleton. I thought that the n° of skeletons allowed in the army was tied to the number of adepts and the likes, but I ended up with a pair of skeletons without having ever built a single adept so far. Maybe the Ritualist was counted as a valid spellcaster skeleton-wise (even though he can't summon them), and together with the imp he invoked at the time that was enough to make 2 skeletons available, or something like that.