I'm sure that's some of it, but it's also a result of lists being pulled ahead of time. Imagine you have 100,000 subs and 10,000 of them are going to get promotion A and then another 15,000 (with some overlap) promotion B. Often, the lists are pulled before the content is ready. Sometimes getting the final approval on marketing emails takes a bit, and so the person who unsubbed when they got email A are already on the list for email B.