Okay, interesting story time (okay, maybe not interesting for all). My brother used to work selling tires and passed this information to me.
Tire treadwear (ie...lasts this many miles) is not arbitrary. BUT it is less than the actual wear time. Why? Let's say the average treadwear for the normal car tire is 80,000 miles. The manufacture will say that is what their treadwear is. But then a different brand now says 90,000 miles. The other brands want to equal or better that, so they say their tire is now rated for 90,000 miles. Did anything change? No...the tire may actually be rated for 100,000 miles, but the manufacturer won't say that; because then to improve the rating they have to come up with a new tire. But, having their tire rated to 100,000 miles, but only claiming 80,000 lets them slowly increase the treadwear rating without actually changing anything. The tire brand my brother sold was rated for 150,000 miles, but only advertised (at the time) for 88,000 miles (or something close to that value).