I think there might be two things happening. When students work in groups they not only share ideas (which help solidify concepts), but you also share answers.

I think there might be two things happening. When students work in groups they not only share ideas (which help solidify concepts), but you also share answers. In my experience (in physics), most students worked together and when you get stuck on a problem you go and ask someone else how to do it. Obviously you end up doing better on the homework (because you complete all of it), and you also understand how to do the problems a little better, so the end result is you get a better grade in the class. The better solution is to do it all yourself. But often that's not possible (because you simply don't know how to do some problems) or you simply don't have to time. So in a per-hour-spent sense, working together and copying solutions probably makes you learn faster, and it definitely gets you better grades on the homework (and consequently in the class). Also, it's much easier to work 14 hours with some friends, then 8 hours in solitude.