Perhaps a question to consider is why a Gun which would not get Gunshield DRM when not mounted on a vehicle (because it is not AT/INF) suddenly has a +3 CE DRM (by vehicle note) when mounted on a vehicle.
I will make a stab at that. A towed 88, as a FlaK gun had a large crew for it's calibre. That would include range taker and extra loaders. The 88, though firing a large shell, could gobble shells like Chris Christie in a candy store, so needed the extra manpower, unlike a
37L or
50L with much lighter shells. As it's a great lump of metal, it's hard to hide and the crew would be spread out more, ammo handlers moving ammo, etc. The BuFla mounted all its crew on the vehicle, so all would get the benefit.
It's hard to explain concisely, but the early ASL developers/designers likely contrasted a smallish AT or INF gun that could be manhandled up to a firing position and also had a small crew, to a standard artillery piece that when emplaced would have pre positioned ammo supplies spread out (to reduce the chance of a single detonation of all your ammo) and a larger crew. That's fine for the majority of cases but there were a few outliers.
An example is the Soviet ZiS-3 76mm (
76L ART) (and it's predecessors). If that gun was exclusively or even mainly used as an artillery gun, that would be fine. However historically it was just as likely to be used as an AT gun. The Soviet ZiS-2 (
57LL AT) used the exact same chassis/mount, only the gun and sights differ from the ZiS-3. Indeed the ZiS-2 (57mm) was
bigger and heavier (1.25 tonne) than the ZiS-3 (76mm, 1.11 tonne) due to its much longer gun. Yet the ZiS-2 gets a small size (+1 DRM) and AT designation (with its lower setup restrictions) and the ZiS-3 gets normal size (0) and ART type setup restrictions. For ZiS-3 guns that were used primarily as AT guns in the historical setting that forms the basis of a scenario, a SSR giving the ZiS-3 a small size (+1) and AT type would be justified.