Since you're mentioning notches and such I'll assume you're talking about the newer ones they sell. FWIW, they can custom make one off ARDs and other parts not worth setting up a line for.
Some do that. I've got a bunch of 'em, they're typically what I use. For some reason some of the caps rotate easier than others; I've got two MRO's and one fits "normal" but one is sorta loose. It's not coming off or anything but the cap will rotate easier. Some of the pieces are made out of aluminum and some out of plastic and that may have something to do with it.
The ARD itself shouldn't be moving, it either threads in or clamps on depending on the model. But the caps can and do rotate so you can open it and flip the cap back so it's out of the way where you want it.
You have it snapped on all the way I'm guessing?
If it's a problem just call 'em, I'm sure they'd make it right.