Ummm...the level of the deck is adjustable too. Exmark has a detailed procedure in the manual for mine which involves loosing the hardware and dropping it down on blocks of wood on a level surface.
But the first thing I would check for is a bent blade.
I'm not saying tire pressure and circumference aren't important, but a bent blade or your deck needing leveling could be the problem as well. Personally I just set all my tire pressures the same and adjust the deck from that point.