Many people weight train to gain muscle mass and strength. Most of those same people will decide on a weigh amount and reps for the day and stick with that weight. The problem arises however, that this causes fatigue and more likelihood of injury. To gain strength and endurance, use varied weights for the different sets, while keeping the step reps the same, as this will allow the muscles to still be working near max capacity, but with less fatigue.
“This imaginary “rule” is likely causing you to leave significant strength gains on the table.”