I noticed this with my truck too. Here is what happens to mine. CHange the oil while it is warm and then put the used oil back into the empty 1 gal containers. Only I put 4 gal in an can usually fit all of the old oil into 3 gals...What is up with that? Well, I got to really thinking about it, because I know it was on the full line on the dipstick before AND after the oil change and the warm oil expands the gallon jugs out. They are pretty fat when they get filled with the old warm oil. Maybe an inch or two wider than when they have the new oil in them. Check out the bottles and see if that is what happened.
-Aaron