If you study for the USMLE you only need to review OMT (and have a magic decoder ring to make sense of many of the questions in general, I hear) to do well on the COMLEX.
However, after preparing for the COMLEX, you probably need to really bone up on biochemistry, molecular biology, biostatistics and psych to do really well on the USMLE. Psych is covered on both, but more in depth on USMLE I believe. And while both cover genetics and immunology, those questions are more highly represented on the USMLE. COMLEX, I hear, pretty much sticks to pharm, physio, path, microbiology, and OMT with a sprinkling of genetics, immuno, and psych.
Not only are the tests non-trivially different in subject coverage, but school curriculums are as well. Biochem, molecular, and biostats are not only more important for the USMLE but are also more emphasized at MD programs (not that all DO schools completely ignore them - some may - but they don't get the same emphasis as say micro or pharm).
Add in the fact that many DO schools specifically prepare students for the COMLEX either through a focused "review" or "integration" type section or course late in the second year, or by arranging for KAPLAN or similar to come to the campus and teach COMLEX review sessions, or both, and you can see how DO students would easily do much better on COMLEX on average than the USMLE.
But remember - that is an average. There are many DO students that do exceptionally well on the USMLE; the ones that realize that it is solely their responsibility to find out what subjects to cover for USMLE and prepare appropriately. I think a lot of students just figure "I'll take both in case I need it for residency" and don't do a whole hell of a lot extra to prepare for the USMLE.
At my school in particular, first time COMLEX pass rates are typically in the mid to upper 90's, while first time USMLE pass rate hovers in the low 80's.
