Dont I know you from ls1tech?
Probably just differences in dyno. But that is a solid pull especially for a mustang dyno. The most common number for the new camaro is 36x to 37x. I have seen some 38x and 39x but those are usually dynapack or dynojet dynos. I havent seen a mustang dyno read out as high as yours for a stock car yet.
One possibility is that your car may have slightly adjusted its computer for when you had the rotofab on...and then when you took it off and went to the dyno it could have still been running different parameters and effected the numbers.
See if you can get another 6 speed car to pull on the same day as you and see what happens.
Also...doesnt your car have a couple thousand miles on it? A lot of motors are known to pick up about 10-15 hp after break in...this could be part of the equation also. A lot of dynos I have seen around here are on cars with less than 500 miles on them.