UTQG is a self certification type. This means that any manufacturer can declare whatever value they want as long as the TIRES WITHIN IT BRAND performs in line with what they have defined on a standard test track (example provided below). UTQG grading is govern by the US Department of Transport.

Example;
Let say tire manufacturer A has 4 tire lines name J, K, L & M and that, each is assigned a UTQG treadwear value of 100 200 300 and 400. As long as the results from the manufacturers' testing show thats M last 4 times longer than J (since M has 400, J is 100), L last 3 times longer than J, and K last 2 times longer than J, they grading is considered correct. All Manufacturer A has to do is to show the test results if the Department of Transport asks. Lets say tire line J achieves 40,000km in the treadwear test.

If Manufacturer B comes and does the same (and have the same rating for their tires J K L and M as A), but for thier tire J their results they got 20,000km. Their rating of 100, 200, 300 and 400 is still valid. Because between the tires within their brand their relative performance is reflective of their grade.

So in other words, you cannot compare the values of a treadwear rating between manufacturers.
Your comment about how the car is driven is still correct and will effect how long the tires will last.

Edit: just to make it clear, from the example: Manufacturer A's tire J has 100 Treadwear rating and lasts 40,000km on a standard test track, but it will perform similar to Manufacturer's B's Tire K that has 200 treadwear rating. All this because there is no standard reference in the UTQG treadwear rating, only relative performance within the respective tire manufacturer. Hence why its meaningless to compare treadwear rating between manufacturers.