Your observation coincides with mine and that of many others, ND4SPD. GT5 Panteras are more rare if we go by the cars offered for sale over time, or if we go by the numbers I have been able to find for the registry (90 GT5 vs. 118 GT5-S).
The relative difficulty in finding GT5 Panteras may simply be a reflection of the greater tendency of the owners to keep their cars locked up out of sight and to never sell them. Or perhaps they were crashed and written off at a higher rate.
Without the factory records we just do not know which VIN to start counting from. My starting to count from 9100 is just an educated guess, and the only evidence I have for the first factory-recognized GT5 being 9250 is the reference in the Matt Stone and Rob de la Rive Box book.
I am starting to be a bit sceptical about the Stone and de la Rive Box assertion for two reasons. First, I have found so many Panteras between 9100 and 9250 with the GT5 body style that it is becoming hard to believe they were all conversions, or that the factory built so many re-bodied GT4s before deciding to start giving them a new model designation. Second, I personally inspected and photographed 9250 in Australia, and it bears no resemblance to the first GT5 that we know had its debut at the 1980 Turin Motor Show.
This is why I have concluded that, for the time being, 240 is the safest number to use as the maximum number of GT5 models that could have been produced. Hopefully more information will be revealed over time so we can be more definitive.