Dear James Smith,
if I understood correctly you want know why different athletes can perform different number of repetitions with the same relative overload weight (in the limit of 85-95% from maximal).
I think that two factors have effect on this issue:
1. genetic predisposition; the innate prevalence of fast muscle fibre and the innate neuromuscular efficiency;
2. the special strength training experience; the best neuromuscular efficiency and the best synchronisations of strength efforts.
Regards the second factor, the training session program, which I proposed to Ariel Schillaci, is finalized to improve the neuromuscular efficiency and the synchronisations of strength efforts.
Yuri Verkhoshansky