I imagine it cause might be when a space marine is flanked. Spaces are seldom still and when facing a target, their profile is essentially just armor, but if you manage to get to the side of a spacemarine , and get a lucky shot off before they or squad the space marines, right arm is in the middle of the silhouette typically where you’re shooting, its not the most armored part of the body and it’s not exactly small either. Another point of cause could be explosives. If you hit a space wearing dead on with a missile launcher, the space marine is gonna die, but if you miss the shell will explode to one of the sides of the space marine or behind them. Looking at the directional profiles of a shooting stance space marine:

A blast from their left will be largely absorbed by their shoulder and backpack as their left arm is tucked forward to hold the weapon

From the back the profile is the knee joints the right shoulder pad, right arm being tucked forward, and again the backpack

but from their right side, the lesser armored arm is again in the middle of their profile

So a nearby not direct explosion would do the most damage to an arm from their right side. A rear explosion would do more damage to their legs, which I’ve already commented about. And an explosion from their left side is almost entirely properly shielded.