While MechWarrior games portray LB-X as shotguns, i'm pretty sure they're implied to fire shells that have proximity fuse to explode just before they hit the target to shower it with fragments. Air-burst shells/grenades basically. This explains its effectiveness against aircraft neatly as well, not very different from WW2-era AA flak cannons. Firing shotgun-style blasts would likely have inferior ballistics for the fragments, reduce effective range.
HAGs are rapid-fire coil-guns. The cluster modifier represents effectiveness of rapid fire in close range and difficulty of keeping rapid-fire weapon aimed at a target. A bit different from LB-X, ignoring the fact autocannons are, well, autocannons...
I don't recommend reading too much into gameplay function of weapons though. It should be noted that autocannons are kinda intended to be rapid-fire weapons. And ultras and rotary ACs are basically really super-fast autocannons. Logically all these should basically function as cluster weapons. But were it so, weapons wouldn't be that different.
(Incidentally, pulse lasers probably should be cluster weapons as well. And standard beam lasers should have the accuracy bonus, at least based on MWO, it is surprisingly easier to keep long-duration beam aimed at the right place rather than a short pulsed beam...)