Well, I can say this much for the gained this or that; that is, some people do gain 5 here or 4 there, but they do not maintain it. If your running a supp and you gain 3lbs, then you come off of the supp, and you start back to training the pre-supp way, then your going to lose it...and if said supp was working for them, or they thought it was working for them then they are not going to train as hard or be as strict on a diet as they were during the supplemental run.
Therein lies the answer ...
I remember when I was running yg/ifg2/blueup...one of the greatest benefits was that I was running something and I also convinced myself that I MUST eat healthy and eat a lot while I was on. This is what I did, plus trained near perfect ... i never missed a workout in the 12 weeks that I ran it. But, as soon as I came off of it, then I started to falter a little. I was not eating AS GOOD, and I WAS NOT TRAINING AS WELL, plus life hit, my mother passed and social events took over....
You have to be damned near isolated from humanity to be able to train for 16 weeks str8 without any reality of life hitting you square in the face.
This is ONE OF THE MANY REASONS why, some people do well ON SUPPS but it never equates to 20lbs of muscle over 2yrs.
One of the other things is that they are not really gaining 5lbs of muscle/lean....some is muscle, water, glycogen. This comprises lean muscle, or is the standard, but not pure muscle/striated. Its pretty easy to gain 5 lbs of muscle/water/glycogen and then lose 3lbs when you come down from a supp and your diet alters, plus the workout...
And let us not forget the one's who are just thinking they are gaining something but are not really gaining anything but a little water...and they are claiming the supp does well because they want to be with the "IN" crowd on the board.