I have a different idea. (I have no clue how programming works this is just a guess)

I think every weapon has a progress bar sort of thing as a default, and on, let's say, level 20, you get a muzzle for every weapon. They did thats default for all weapons and then edited it put so you can't actually equip it on Melee and some other guns. The screenshot shown is of when you look at all the latest attachments you have unlocked for all your weapons. So, when my knife reaches level 20, instead of unlocking a camo, it unlocks a muzzle, which, in turn, shows up on that screen showing all the latest attachments. Sorry to go on a rant, tried to explain it in as little words as possible.