There are a couple different reasons for this in my mind:
1. There are fewer teams at each classification the higher you go. This means that the talent pool is likely to be more consistent. For example if you take a bottom of the rankings C team against a top 20 D I would argue that the D team should have a fairly decent winning percentage. The talent at these levels is just not consistent.
2. The second reason is that the higher you get classification wise, not only is the talent better but I would argue the performance is more reliable and that the participants tend to be more focused. While it happens to the best of us, the higher in classification the less susceptible players should be to mental letdowns and not taking an opponent serious. If you have a C team playing a backyard tournament it is highly more likely that a lower team beats them due to the C team due to mental aspects than it is dealing with an A, AA, Major team at a conference event. Again, not saying it doesn't happen its just less frequent.