Something you run in to is the "due date" on a WU as well. BOINC will usually try to complete a WU with a sooner expiration date than one with an older date. So if POGs (for example) has WUs that are due back to the server sooner, and your AP WUs are good for maybe a couple weeks or something, then BOINC will keep burning through more and more POGs until the AP WUs' due date gets closer.
Hope that makes some sense.