I think we all know people like that, but the trends are what they are for a reason. Just for fun (I have time to burn), I surmise that it is the way that it is because there are three types of people who apply to medical school:
1) People who are very skilled test-takers, who may or may not really care for the material that they're learning. These people likely make up the majority of medical school matriculants at allopathic programs. For this group, their MCAT scores indicate how well they can prepare for an exam with a lot of material, and thus it is not surprising that there would be a correlation between MCAT, Step 1, and Step 2 CK scores.
2) People whose testing performance depends on how engaging they find the material. This group will include the people who score 24's on the MCAT and then blow away the Step exams. It will also include the people who fail out of medical school. Depending on how their baseline ("uninterested") testing abilities are, you'll find this type in DO and MD programs. With increasing competition, I would wager that they'll enter DO programs in greater amounts.
3) People whose test-taking (and perhaps academic) ability is low overall, regardless of the material. This group will most likely be forced into DO (or international) programs, if they're able to make it into medical school at all.
Actually, the last group (and the entire grouping in general) could be verified if we have data on the USMLE scores by IMGs. After all, they don't have OMM in their academic curriculum, but it is stereotyped that they're the "academic undesirables" in the admissions process. How do their scores compare to those of DO students?