So the TREADWEAR grade will not tell me for certain how many miles of tread life I can expect?
Correct. It is not a measure of tread life. It indicates on that, under the same general conditions, one tire can be expected to produce more mileage than another, not what the mileage will be. Again, actual tread life is strongly influenced by where you live and how you drive. In some parts of the country, tires wear out faster. For example, the local rock used for road construction is rougher in some geographical area than in others. The same tire might give 20,000 miles in one part of the country and 30,000 in another, but a tire with a higher grade should produce better tread life than a lower graded tire in either locality.