Take example School A and School B, each with 5000 applicants, 500 interviewees, a matriculation rate of 50% on May 15 and target class size of 150.
School A gives out 200 acceptance offers, yields 100, and gets the remaining 50 off the waitlist by ensuring each one commits before accepting (or has at least demonstrated strong interest in the school). Overall acceptance rate is 5% (250/5000), pre-5/15 interviewee acceptance rate is 40% (200/500).
School B gives out 320 acceptance offers, yields 160, and then incentivizes the other 10 to defer admission (or some go elsewhere from WL offers). Overall acceptance rate is 6.4% (320/5000), pre-5/15 interviewee acceptance rate is 64% (320/500).
School A (while a bit exaggerated) is actually a fairly common practice from what I remember while applying -- the more update letters, interest letters, and letters of intent you send, the better you stand in late May. However, Michigan's approach is like School B, hence the excellent post-interview acceptance rate compared with other places, since our admissions team is not as concerned with keeping our overall acceptance rate as low as possible (not that there's anything wrong with doing so). Thus, Michigan has not needed to use the waitlist in years. Your trip to Ann Arbor (IS or OOS) is a great investment, since you have a great chance of gaining an offer a few weeks later.
FWIW, of the 364 accepted last cycle, 198 committed to coming on May 15 (54%). By matriculation, the M1 class enrolled 177. A good number deferred until Aug 2015, and some took WL offers elsewhere. Technically still overfilled, but they added extra mailboxes 😛