I'd be willing to bet that the tire companies perform their own testing, and are not sufficiently policed about their testing, and their methods, and how the numbers are calculated. As a person who has conducted very stringent, third party, tests, I will tell you, people will cheat on these things all day long if you let them. Especially anytime the marketing department gets involved.
My experience is with power quality, and not tires, but there are parallels as far as the testing is concerned. If I had wanted, I could have designed my tests to generate the required data, and made things pass successfully, even though things were not up to snuff. Unless there is stringent, monitoring, by reputable third party elements, tread wear is meaningless when comparing one company's tires to another.
There are just too many variables to overcome unless the tests are apples to apples. Even if they use an ASTM standard, there needs to be more in common between the tests to make the tread wear numbers mean anything other than comparing one tire to another within a single tire maker.
