i recently accomplished 3 reps on 80 kg when the bar was loaded with 2*20 plates and 2*10 plates. the next week i went to the gym and this time loaded the bar with 6*10 plates (80 kg total weight again) and this time achieved 8 reps quite comfortably.

the heavier plates seemed to make a difference - ok it was a week later but still would this account for 5 reps! which has never happened before.