Tear wear confusion

i had the car in for service today. As i was told last time (May), i need new tires (i have not been driving this car since June except for rare short trips, using another car temporarily). I was told that three of the tires are at only 2 tread depth. He said that one of the tires is new and is at 10 tread depth. But i did not get any new tire. I got all four in 2/09. There was no explanation about why only three are worn out.

The tires are Good Year Assurance Triple Tread, i replaced the factory tires with them when i bought the car, i'm the second owner. The car was at 29K when i got the tires and is now at 65K. I got the tires at Discount Tires.

Is there a sensible explanation for one of the tires appearing new after almost 3 years of driving that wore out the other three? Could this have anything to do with having a non-Toyota dealer tire guy set the sensors (Discount Tires)? That sounds unlikely but i am very puzzled. i am glad that i only need three tires instead of four, but would like to know why.

on the question of the tire warranty which is supposed to be to 80K miles, i read on another thread that this would probably not be hononered if the tire wear pattern suggested underinflation. But if they were inflated according to Toyota factor specs (which seems way too low to me at 35F and 33R, feeling too squooshy), how can that be considered underinflation? (i called Discount Tires, the guy said he would discount the tires $10 each because they wore out. he had no explanation for one tire being 'new,' he said i probably got a new tire, but i didn't. i would remember doing that. Once, a tire was flat and air wouldn't stay in it, i took it to Discount tires and they patched it).