I am wondering whether the chance of getting accepted off a waitlist is greater for graduate programs and particularly for doctorates.
I am thinking that overbooking might be common for bachelor’s degree programs. However, for a PhD or DVM or MD program if they have very few positions (eg, some PhD programs may have a single digit number of acceptances each year), they might not be able to overbook.
A number of years ago one daughter was accepted off a waitlist for a DVM program. My recollection is that she had just decided to go somewhere else (to her #1 choice) and therefore turned it down quickly figuring that another student was likely to get good news that mattered to them.