I don't think variability is a uniquely DO problem actually. What we really lack in our rotation is a structure that protects teaching. I think the general attitude of DO schools towards their students is more problematic and is just expressed via poor quality control on rotations where students get used as pure scut, and get minimal education.
It also shows when you compare time off during 4th year for boards/interviews. I was talking to an intern at one of my aways and she was talking about having 12 weeks off for boards/interviews in 4th year plus a research month. Compare that to my school that has the asinine rule about you can only miss 2 days a 4 week rotation for interviews, and gives 4 weeks total for boards/interviews. Its just a different mindset. DO schools are very reactive and don't see the need to change anything till it really doesn't work (i.e. students stop matching/placing at all) vs USMD are very concerned with how, where, and when they match.
The relationship with MD schools and their students is, in general, cooperative vs DO school where its often adversarial. And I think this trickles down into rotations as well. Our schools (DO) are not actually committed to our success, they just need somewhere to stuff us, and many times they don't want to help out if your trying to do anything different. If you tell your MD school you want to ENT, at most programs they will hook you up with a mentor, arrange rotations at the home program, try to set you up with a research project. If you tell your DO school you want ENT, you will get a 'good luck' and then they will try to restrict how many ENT rotations you can do while forcing you to do 5 primary care rotations during 3rd year.
Anyway, I am generalizing. This is obviously not true at every DO or every MD, but it is something I noticed. And I have gone off the deep end, so its time for me to go do some work.
The only hope is the ones who did it before you and got what they wanted. We just have to overcome, its all that there is.