I had a similar problem with my Craftsman T140 when I first got it. I must have checked the left to right adjustment a dozen times. Every time I did it, it looked nearly perfect while in the garage on a nice, level concrete floor, but when I went out and mowed, it was still slightly off. I finally ignored the normal and usual method of leveling and deliberately gave an extra 1/2" to the lower side. It worked. I got that tidbit from an old time mower repair guy in this area. He said that a lot of the MTD and other middle of the road mowers just allowed the deck to pull "cockeyed" to one side under load because of slightly inferior construction materials. That's not a jab at any of the machines or their owners. It's just an observation an old timer had made. It worked for me. Just compensate for the amount of unevenness and set the deck slightly slanted.