Well, in previous CODs with without variants and lesser amounts of SBMM you could use really bad guns against really bad players if you were better than the general population (lets assume most people on this sub are "better than average") .
So I believe that SBMM has lead to having to use the "best" weapons in order maximize your skill cap (Maximum capacity for success in this game). This is the only COD that if I were using a 3rd or 4th best gun in a category I would completely get demolished. It is because I am being paired against people of my skill level.
So if I give myself a built in disadvantage (not the best gun) then it is easier detected because in theory I cannot overcome my skill cap. Basically, if I had bad kids & good kids in the same lobby, unbalanced weapons wouldn't be as detectable because they would "melt some people" and suck on others. Which I would then attribute to myself because if it is melting some of the time, it must be me.
I can use the AMR9 if my friend hosts a lobby, and drop 20-30 kills in TDM/KC, but if I were to host? If I don't use the ASM1/BAL/HBR I will not have the same results.
So if I use just the results from when my friend hosts (.85 KD) then the pre-patch ARM9 is perfectly balanced and doesn't need a buff. If I use the ARM9 in my lobbies (1.3+ KD) then it is a completely useless weapon.
it's all about perspective.
Now factor in variants on an already extremely obvious lack of gun balancing. Now ontop of only being able to use the "best of" variant for the few usable guns in this game, you now have a perceived disadvantage because you are lacking those guns. It may not be real, but perception is key. If you see yourself failing, you will fail. When you fail you will blame whatever is the nearest "thing" you think can be justified for your failure. It is human nature to hate failing and some will blame whatever they have to in order to not admit it was them.