This comment was created by combining multiple isolated and unrelated facts to try to make a cohesive argument. If it turns out to not be true, i apologize, but i think it should hold up fine.
Your argument has a flaw in that there is a very large excess of ACGME spots. Very, very, very large. And for that matter the percentage of DO students who go into the AOA match increases every year. (at 51% in 2010, up from 48 and 49% recently). The giant excess of ACGME spots are in 'less desirable' residencies though. But, coinicidentally, EM is one of those 'less desirable' residencies. Despite it actually be a good residency for a bunch of reasons, its among those residencies where there are a lot of extra spots to be had across the country.
So with all of this, my only assumption is that 70% is either the pre-scramble match rate for all resident applicants to EM (MD and DO). At which point this is a non-story. Or it could be that 30% of DOs have no clue what they are qualified for, as they are applying to only overly-competitive spots when plenty of spots within their same field go unfilled. And those spots are likely available in *somewhat* local areas to wherever they applied.
and a point of information, all of these facts are pre-scramble. As neither the ACGME nor the AOA collects post-scramble data for these statistics. The only post-scramble data available comes from the federal government, which makes no differentiation between MD and DO students, so the specifics cant be extrapolated from that data.