The first number is a rough indication of how many miles you might expect to get out of a tire in average, everyday, normal driving.
But there is a fair implication that a high treadwear rated tire is typically not as grippy as a low treadwear rated tire. Tires are something of an engineering compromise among many different criteria, and to get a little more of one you typically give up a little somewhere else.
Sometimes a little marketing gets involved at the low treadwear end if a mfr suspects if he advertises a true 200TW tire at 200TW, he will lose sales to the mfrs who are advertising their 200TW tires at, say, 140.
Here is one such instance, and the "rerate" to 200 (with no changes in the tire) is to suit there being more competition events now that require 200TW-rated tires. It is legal to advertise a lower TW rating than actual, but TW numbers may not overstate the tread life. Of course some will "game" that kind of rule if there is benefit in doing so.
FWIW, you ought to be able to get 10,000 - 12,500 miles per 100 treadwear in even fairly enthusiastic street driving if you maintain inflation pressures and alignment that suits that driving and don't get into wheelspin except truly unintentionally. Involvement in autocross, drag racing, or road course track time will of course reduce that estimate.
Norm