To start off, each lifter reacts differently to training as any person reacts to any type of stimulus. Some people believe they gain better under one type of rep scheme over another. This could be because of many reasons. I've read such high-tech explanations that certain lifters can activate more muscle nerve fibers simultaneously than other lifters. At least this sounds good.
Some lifters have mental aversions concerning singles.
Liters who can deal with max weights and new personal records on a consistent basis, obviously stand a better chance of extracting more from their use. Another factor is comparing apples and oranges. By that I mean that performing rep sets is much different than doing single rep sets. With rep sets, the weight is lighter and has a greater margin for error in their execution. If you rep out a set of 5's, getting one out of the groove will seldom stop you from getting at least 3 reps.