Where did you get that statistic from? It really depends on the school. DocToothache has thread detailing the ratios you are asking for. You can also look up the latest stats from the ADEA guide.
Generally speaking your chances of getting accepted are higher when you interview Pre-Dec than Post-Dec. I would say this is more so the case with public schools than with the private ones.
As a rule of thumb...
Public schools tend to be the schools that are most competitive because they are closer to home and tuition is usually less. Therefore the first wave of acceptances from these schools are usually NOT turned down by interviewees. Because most spots in the class are being retained by these interviewees, there aren't that many seats left to be filled by post-Dec interviews. Interviewing post-Dec could then mean fierce competition for a spot in the class or maybe just for a spot on the waitlist.
When expensive private schools give out their acceptances, on the other hand, many of the people who got into their state schools turn down their acceptances/withdraw from these private schools. This frees up a good amount of seats for post-Dec interviews and even lessens the applicant pool altogether. So interviewing at a private school post-Dec means you still have a decent chance at getting into the program. Hope that makes sense!