Yup road surface, driving style, inflation maintenance, rotation (or lack there of) all affect it. The treadwear rating is done by the manufacturer so it's most relevant comparing between models of a like brand in relative fashion. That's why it's not rated in miles. It's somewhat useful across brands but Michelin, Continental, and Yokohama might not necessarily rate it the same. Still a 600 or 700 tire is almost certainly going to last longer than a 300. But twice as long...... maybe.
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Tire ratings are pretty complicated. They run a prescribed loop in West Texas with a convoy of vehicles running both the tire to be tested and a NHTSA "standard" test tire, rotating every so often. After 7200 miles they compare the tires.