Well the point is that there may be the same number of U and T available when a flight goes on sale, but the lower class will always sell out before the one above.

If people are searching by lowest fares (99.999% of flyers), they are going to buy T until T sells out and gets to U9T0.

(Yes, someone is going to point out the almost impossible scenario where at U9T5 two groups of people come along wanting blocks of 6 tix and are pushed into U, making it sell out before T, but it's extremely unlikely.)

It's not a HUGE thing, but it is certainly a negative.