Thanks Paul. Your comments made me look into this further. Here is the info from Wikipedia.

Treadwear rating

From Wikipedia, the free encyclopedia


The Treadwear Grade of a tire is the numeric portion of the Uniform Tire Quality Grade Standards (UTQG) that are printed on the sidewall of a tire. These standards were enacted by the National Highway Traffic Safety Administration (NHTSA), which is part of the United States Department of Transportation. Higher treadwear numbers indicate that the tread of a tire, and hence the tire itself, should last longer.

Methodology
The wear on tires that are being tested ("candidate tires") is compared to the wear of Course Monitoring Tires (CMT), which are sold by the NHTSA at its UTQG test facility in San Angelo, Texas. Both types of tires are mounted on vehicles that will be driven in a convoy during the test, thus ensuring that the candidate tires and the CMT tires experience the same road conditions. The convoy, typically one of four or fewer vehicles, will drive 7200 miles on public roads in West Texas. Candidate tire wear will be checked during and after the test, and compared to the wear on the CMT tires from the same convoy.
The first CMTs were commercially-available Goodyear Custom Steelguards, and Goodyear Tire and Rubber Company produced all CMT tires from 1975 until 1984. From 1984 to 1991, the CMT tires were produced by Uniroyal. CMT tires are now "specially designed and built to American Society for Testing and Materials (ASTM) standard E1136 to have particularly narrow limits of variability." 1.
Treadwear Grade Number

The Treadwear Grade describes how a tire manufacturer views the wear of a given tire. In theory, this means that a tire with a 200 grade will wear twice as long as a tire with a 100 grade. However, tire manufacturers are not under any obligation to grade a tire based on the test results, except to say that they can not overstate the grade. This is enforced by NHTSA requiring documentation to justify any assignment of a grade on a tire.
Please note: As Course Monitoring Tires have changed, their treadwear grades have changed to numbers considerably higher than 100. As a result, it would be incorrect to say that a tire with a treadwear grade of 200 gets twice the life of the Course Monitoring Tire.


Limitations

Notice that the treadwear grade is a ratio and not a mileage. This is because there are a lot of factors that determine treadwear rates and most of them are a function of driving conditions and operating environment, and not the tire itself. As a result, actual tire wear will vary considerably within the same tire line. The phrase "Your mileage may vary" applies here.
The assigning of UTQG grades is done solely by the tire manufacturer. In many cases, this has resulted in the UTQG grading system to be more of a marketing tool than was originally intended.
It is legal and permissible for a tire manufacturer to give a particular tire line a lower treadwear grade. For example: If the highest treadwear grade in a manufacturer's line up is 600, then a tire line with a lower treadwear test result might receive a grade of 400, instead of the 480 it could possibly receive.
Also, it is common for tires whose treadwear grade is of little commercial value, such as "DOT" racing tires, to be assigned extremely low values - sometimes even zero.