Got a 66" Exmark Triton with the upgrade. Cuts great, however when I go to sharpen the blades the two on the outside ends of the deck are always duller and more dinged up than the middle blade. Is this true of all models? What causes this, the positioning of the antiscalp wheels? Just a dumb question, I always rotate the blades to keep the wear even. Or is this indicative of some type of problem?