The recommendation for lower reps is because lower reps allow you to work without the effects of fatigue. This has several advantages. Firstly, it creates a safer weight workout. In my opinion working with 80% of 1RM for 1 set of 8 reps will have a higher injury rate than using 95% for 2 reps and certainly MUCH less than using that same 80% for 4 sets of 2. This is due to the causal effect of fatigue on inury. By controlling fatigue you can also work on the things that the weight room is best suited for: strength and speed development.
The lower reps I’ve recommended doesn’t however mean that you need to be doing high intensities / loads. In fact, I wouldn’t recommend this for GPP. This is where the high sets recommendation comes in. By increasing the number of sets, you can still use a lower rep scheme (and get the benefits listed above) and still have the high volume necessary for GPP. An additional benefit is that doing a greater number of sets for a limited number of exercises leads to better strength gains than a greater number of exercises with fewer sets.