I noticed this on my old phone (Samsung Omnia SCH-i910), and I think I'm noticing this on my MAXX as well, but I'm not sure. On my old phone, my vehicle charger would appear to charge but seemingly the charge would not "hold" as well as if it was charge from a wall plug.
Is this true for phones in general? Could it also be that while charging through the vehicle charger, I'm several running apps, thus draining the battery more versus when I plug it into the wall, have it turned off and turn it back on when charged and begin using it again? I don't know if that would apply to my old phone, though, considering it had nowhere near the capabilities of the MAXX, thus I wasn't running that many apps on it, period.