It’s ultimately just tolerance stacking.
For better or for worse, the L4/Universal/USGI pattern has been around for over twenty years and manufactured by countless companies, from the earliest plastic PVS-7 Rhinos to the latest L4 and Norotos Hyper mounts, and they’re all supposed to be interchangeable.
I’ve had mounts and shrouds from all different companies—some combinations that need to be hammered in, some that feel like they’d fall off with the slightest bump.
Obviously out of spec mounts can happen in the course of manufacturing, but the spec tolerances are inherently fairly generous due to the longevity and variety of manufacturers over the years, so to a certain extent it can sometimes simply be luck of the draw.
Ultimately it’s part of the reason for some of the various semi-permanent fixed mount designs as well as the Wilcox “rail shroud” used on the GSGM/DPAM as well as the G24R.
Luckily, there are a number of ways you can adjust tension on mounts besides simply swapping them and finding a good fit through brute force, including using shims of various types (thin foam/rubber dots can work, as can soft sided Velcro), as well as using bungees to retain tension within the whole assembly.
It is wholly possible that simply using another mount or shroud might produce a totally different fit, but it’s hard to “blind match” with a 100% guarantee of perfect fit unless you have a lot of different components to mix and match, though like-manufacturer combinations tend to have a higher chance of fitting well since they’re at least using the exact same tolerances in manufacturing, but again, in the end it’s a symptom of having to manufacture interfaces that will fit such a wide range of potential parts that have not always been held to as tight tolerance as they are now.
FWIW, I have enough mounts and shrouds at this point that I could brute force check and match them, but I never bother and have never had an issue.
~Augee