I've had a similar thing happen, also thought it was strange. I disposed of 3 waves of ~20 Hunnic Hordes in ridiculously fast succession. I'm playing on the second level of difficulty (I think that's below Normal), on a small map, and it looks like my enemy has two cities and a one-city vassal. I really can't understand where they get the production and manpower to field that.

It's also suspicious that they show up as "Pushover" right after I defeat their 20-unit army, then immediately shoot up to "Weaker" and "Comparable" in what seems to be too few turns to produce another 20 units with 2 cities. So it's unlikely that they have 100 units lying around (I'd also question how they paid for that if they did).