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A couple of additional thoughts:
First is that the steel belts in tires are indeed cables, but they are at an ANGLE to the circumference. In other words - they are NOT hoops and they can stretch! That means that while they RESTRICT growth in the center of the tread, they don't PREVENT it.
Some tires have cap plies - usually made of nylon - and while those also restrict the growth of the tire in the center of the tread, they ARE (more or less) circumferential. Needless to say, tires with cap plies would be less likely to wear out at the center than those without.
HOWEVER, tire pressure has a small affect on even-ness of wear. There are other things that have more affect. For example, it is known that "spirited driving" tends to wear the shoulders out. So if you go around corners quickly, and use high inflation pressures, there is a cancelling effect on even-ness of wear
So to answer the original question: Why the difference between the vehicle manufacturer's specification and the max on the sidewall?
Well first, the tire manufacturer doesn't know what vehicle the tire is going on, so what is written on the sidewall is an operational limitation. The important thing to remember is that the load carrying capacity of the tire and the shape of the tire's footprint is based on normal load vs inflation pressure usage.
The vehicle manufacturer sets the inflation pressure based on a lot of different parameters - and ride quality is certainly one of them. But ALL the vehicle manufacturers work within a small range of load vs inflation pressure values - ones that are easily characterized so the tire manufacturers set up their tires for those conditions. I am of the opinion that using inflation pressures outside the vehicle manufacturer's specification has inherent risks - and ones that we just don't have a good handle on.
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