Well, maybe you ought to call Hustler and tell them that they are wrong and that you'd be happy to straighten them out on their specs.
You have older SZ's. Maybe they did at one time have higher BTS's on the 72 inch version, but I doubt it. You are constantly promoting 72's over 60's etc, and that's fine by me, but just because you
think that they're the best doesn't mean they are, or that they have the highest BTS. Each size has its own place. No one size is best.
You still havene't proven anything other than saying that you think
they have the same sized pulleys, spindles etc. Only a 1/16" difference in pulley size at any one point on the deck or PTO could easily provide the difference in BTS posted by Hustler.
The thing is, you claim(ed) that the 72's have faster BTS, but none of the manu's that post the specs agree with you. Either they are the same for both deck sizes, or their 72's are slightly lower. Now I suppose you are going to tell us that they are all wrong, and you are right.