Like ScottTheEnginner said, I don't think it has to do with forms. I think it has to do with the specific agent and how they are feeling that day. I also agree with ScottTheEnginner that some of it may be on the fringes of legality according to NAFTA, on both sides. I have heard some bad stories about going into Mexico for work also but I have never had an issue.
Just to bring this back the other way a bit I have also had a case where a Canadian Customs officer probably could have rightly turned me around and sent me home but instead helped me get into Canada by creatively interpretting my situation. So there are some good cases to go with the bad.
Keith