Inferno tactics.

Ah, a minimum Dragon Utopia... Then that explains it. 10 demons from the spearmen. You had 21 dogs. That's another 15 demons. Now for the imps. 61 + some amount with the hero. Let's say together it' same as 105. That's another 3 demons. So far that's 28. Gogs. You have 33 of them + possibly whoever came with the heroes. From those, you can make 12 demons. Well, 14 let's say.
In total, that makes 42 demons. I counted the external dwellings of gogs and dogs, as you can see. Fine, maybe you didn't slaughter all 66. Maybe you bought another 8 pieces. Well, in general, it almost adds up...