If the blade tips are worn rounded this would cause this problem as it would the same as the blades were too short. Normally the blades have a straight line cutting edge but blade tips take a lot of abuse as they make first contact with anything including rocks.
This another reason the decks are shifted forward on the side opposite of the side discharging. Norm is right about the deck blades being offset and when the blade are correct they over lap in the direction of the cut.
Some manufactures are even timing the blades so even longer blades are used. But this requires a timing belt and blades timed 90 degrees off from each other.
Normal offset blades are usually with 1/8"-1/4" of each other when align tip to tip depending on the amount of offset. I have at least one set blades that actually hit each other which required a little grinding on the tips to stop it. Of course they were after market blades so probably was just a tad too long.
