Tire manufacturers are subject to spot checking of tires and their tire ratings, so they need to be careful in overstating their treadwear rating. Aka if they claim a 300 and are really a 200 they would be in trouble, but the same designation doesn't go the other way.
A tire could actually be a 300, but they may use 140 for marketing purposes, capitalizing on the fact that people think the lower the number the grippier the tire, as was posted in your original question.
It's really a bad system, and not one that will correctly give you a whole lot of exacting info on how good a performance tire is, nor how long the tire will last.
My last set of 300 rated tires were toast in less than 10k miles (6 months).. due to my large right foot.. so if you're driving hard (not talking burnouts, just driving a car hard will wear tires significantly in the rear) you're going to blow through tires..
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