The reason is the dynamics doesn't give an acurate real world load on the rollers. The mustangs does. The mustands rollers are connected via a belt, so they must run at the same speed. the dynamics isn't and that's one reason you can't run a 4EAT on it, because it have 90/10 tq split, and the numbers just won't work out. On the mustangs, it would work out fine, since both rollers move at he same rate and the heft of the rollers renacts the load/resistance of a street surface pretty closely.
I go to www.gruppe-s.com
for my dyno runs.