OK, so it sounds like only minutes are going by before you are getting the test. Like Deanie said, as long as it isn't too long, the food wouldn't have raised his numbers much if any. By 30 minutes, some of the number will "include food".
Is there any way you can remove the food a couple hours before the PM test? Free feeding during the day is fine, but if you can manage to not let him eat for a couple hours before the PM test, your numbers will make more sense. It removes the "how much of this is food related" variable from the picture.
Carl