Yeah , thats just wrong. I have seen as recently as yesterday (when playing HoF anyways) that certain guys would for some reason drop down to their 50s/60s in stamina while the other guys on the court with them were still in there 80s and "fresh".
Playing a five man lineup for the whole game (on 5-6 min quarters) is just not feasible if trying to win. Guys/guards have high stamina and might be able to play for a long time but bigs and centers really slow down after about 4 constant minutes.