Hm, I thought this is the normal case. I only tried this one time and ended up being slaughtered by a horde of bandits. Can't remember if that was in Sturgia, but it felt unbalanced. Like you said, I killed a dozen on my own but my companions got slaughtered in no time leaving me with several enemies on my own, too many to deal with. I made a way around the back streets from then on, guess it's time to try this again...
[EDIT] Okay, I went to Husn Fulq and cleared the backstreet, and the second wave of bad guys spawned in town with only a handful of my men at my side. Maybe it's random if you can fight them outside with all of your army, or it's a bug.