2005 Toyota Corolla Wheel Alignment

Through the years, I have taken my car in for wheel alignment on several different shop, usually this is done every 5000 or so miles or when I feel that the car is no longer tracking straight. What I do not understand is why is it that on one shop, the alignment changes he made can last longer than the rest of the shops. When I bring my car in to this shop, the alignment changes he made can usually last for a good 15,000 miles or so. Daily commute is basically the same route everday and the car has not been driven into potholes.