Stendec - not uncommon for blokes to pass UKSF having previously failed P-company or AACC.
Also, it was not uncommon for UKSF recruits to have to do all the P-Company tests during their continuation training (particularly if the DS were mostly ex-para) and there were certainly blokes who had romped through the selection walks who couldn't complete some of the P-Company tests, even at the high level of stamina fitness post SF selection, because they didn't have the power fitness required for P-Company.
Lots of factors come into play - different body-shapes, different mixes of power/stamina, different levels of tolerance to cold and lack of sleep, fear of heights, upper body strength - it is basically horses for courses.