The problem came from using custom calendars in some of the projects because the resource pool has the standard calendars and when projects increased hours, it defaulted to the pool calendar.
I believe (hope?) I have fixed the problem by unlinking all projects from the shared pool. I made sure that the "ignore resource calendar" box was checked in every project, and when I relinked everything to the shared resource pool, I made sure that the "sharer takes precedence" over the pool calendars.
So far, so good. Thanks for your help. I do appreciate it!