Many of the good sets and reps scheme automatically fall in that rep zone.
However I don't buy the "total reps" theory, I think the fact that many good sets/reps schemes fall into that zone is a random thing.
If total reps what the key thing then all that would matter is getting the reps in. So 25 sets of 1 rep with your 6rm or 4 x 6 with your 6rm would yield the same results... which is far from being the case.
Heck even rest intervals can play a role...If I do 6 x 4 with my 6RM with 3 minutes of rest it is actually fairly easy because I do not accumulate fatigue from set to set. But if I do the same 6 x 4 with my 6RM and only take 45 seconds of rest then it becomes hard because by the 3rd set I have accumulated enough fatigue to make the sets hard. So in both cases I have the same total number of reps but I'm creating a different training effect.
And what about programs that have been proven to work but are way outside of the range? Like German Volume Training (10 sets of 10 reps for 100 total reps)? Or my layer program where you have a total of around 75 reps on an exercise with weights ranging from 70% to 95% (or more)?
And what about supersets and triple sets? Let's say I'm doing a triple set of:
A1. Dumbbell shoulder press for 10 reps
A2. Dumbbell lateral raise for 12 reps
A3. Dumbbell front raise for 12 reps
I can only do 1 set because 2 will get me to 68 reps. Does that make sense?
And finally the 25-50 reps rule is applied to an exercise. But what about the number of exercises hitting a specific muscle structure? Is it 1, 2, 3 exercises per muscle structure... and that makes a HUGE difference.
See I think that some people are comforted when they can hold on to one precise rule. It's our instinct to try to create these rules to feel secure that what we are doing is the right thing. But I do not believe that such rules exist with training.
When Zatsiorsky talks about "a fiber that is not fatigued is not being trained" he refers mostly to the single bout effect: meaning that if you do not fatigue a muscle fiber during a set, then that set doesn't train that fiber. So when using the repetitive effort method (sets of fairly high reps with moderate weights 6-20 reps+) his point is that you need to take the work sets close to muscle failure for that set to fatigue as many fibers as possible.
Now, as I mentioned earlier, there is an accumulation of fatigue from set to set. If the fibers recruited in set no.1 are not fully recovered when you start set no.2 then you will have to bring in more fiber into play and eventually you will also fatigue/stimulate these fibers.
But I do not think that tracking total reps for an exercise is the be-all end-all element for the reasons I mentioned about. I think, when talking about HYPERTROPHY it is more important to focus on how much fatigue was created on each set then on the total number of reps.