Maybe the edge of the leg isn't all the way in the body. Sometimes ...... tjah .... sometimes boolean operations behave like that. Booleans are unpredictable.

But if you got that far in the 3rd tutorial of the cartoondog,... u'll should have noticed that the author/instructor Kurt Boutilier also faces the same problem. 17min in that same tutorial he shows you how to go around it.
Use split polygon tool, delete faces, combine the leg and body then merge vertices to close up the hole.