Ahh...I think I've encountered the same thing in a 88 Bonneville I used to have. Any governor I've encountered in a car (and I've explored alot of them...Honda Odysey 3.5L, Olds Cutlass 3.1L and 3.4L, Olds Intrigue 3.5L, Buick Century 3.3L, Olds Silhouette 3.4L, Toyota Camry 2.2L, probably more)....the 88 Bonneville and an 85 Olds Cutlass 442 were the only ones that I ever topped out and just hit a plateau...no abrupt cutout.
I'm to scared to try anything over 75mph in my 90 Lesabre, as the only thing that is holding it together is rust.
Anyway, I really don't have an answer to your concern. If your really eager, you might try installing a 120+ mph speedo to see where you really are at top speed. Have fun!!