Almost all bodybuilders are firmly convinced that they are training very hard but, in fact, I have never seen a single bodybuilder who trained as hard as he should; almost all bodybuilders confuse amount of training with intensity of effort and, too, most bodybuilders are simply not willing to train in an actually "hard" manner.
They believe they want to believe that they can produce the same results by training more by performing more sets, or more exercises or training more often. That is at least understandable, since it is certainly more pleasant to do an extra set than it is to do the extra two or three repetitions that should have been performed at the end of the first set but such wishful thinking won't change facts, and no amount of extra sets will make up for a lack of the final, actually productive, repetitions that most bodybuilders skip.
In a set of 10 repetitions leading to a point of failure during the 10th repetition, the first seven or eight repetitions are simply preparation, they do little or nothing to stimulate growth; but they do use up part of the recovery ability that makes growth possible.
Thus, if the last two or three repetitions are not performed'then the set was wasted; worse than that, it actually did some harm to your progress'because, while it did nothing to stimulate growth, it did exhaust part of your recovery ability.
Arthur Jones, Iron Man, January '72, Vol. 31, No. 2