I don't have a catchy acronym, but I know something about this. Programs are funded for a certain number of residency positions, which generally means they have the same number of positions per year. However, many people do pathology Ap/CP combined while others do only one of them, which cuts down the residency years. Thus, when one of these folks leaves after 3 years, there is an extra spot. So, some programs will try to fill a spot with a more senior resident who wishes to transfer in, if they wish. Others will take an extra person for their PGY-1 positions. Some places don't want to try to have someone join their program half way though. It is all done to keep the maximum number of funded spots. A program with 16 spots might not have 4 residents per year, because of this. Might be 5,4,4,3 for example. Or it might be 2,4,5,5 because a previous class was large.
There are also variations on how many spots are offered through the match because some programs will offer some spots outside the match to non-AMG's who have done significant clinical work, research, etc.
I am not sure why it is making you nervous. It is a natural cycle of things. Some programs will have fewer spots to offer on certain years, but other years will have more spots. I guess it might be tough if you have your heart set on a certain program and they take fewer people that year. Most programs tell you on the interview day (if they don't, they will tell you if you ask them) how many spots they will have in the match and why that is different from previous years.
