The question comes up because your allocating 4 original wounds to models then the wounds on those models double if they are not saved. So you only would roll 4 saves for 4 units then they would be doubled for each guy does that mean the automatically allocate to a different model?
Okay, now I can see what the debate is. And I'm reading the rules a bit more closely now too. (This is why debates come up, because this rulebook is dense and poorly organized.)
Page 26
"If the unit consists entirely of models that are identical in gaming terms and have multiple wounds, then take all the saves for the unit in one go."
Basically, wounds on the Scarab squad skip the allocation step, because they're all the same.
Continuing on...
"Once you have determined the number of unsaved wounds suffered by a group of identical multiple-wound models, you must remove whole models where possible."
So the Vulnerable USR interjects itself right after determining the number of unsaved wounds.
Determine the number of unsaved wounds, double that number, then remove whole models where possible.
The correct example...
In the Scarab example, let's say there's a unit of 10 Scarab bases. A Flamestorm cannon drops its template over 6 of those bases, so 6 hits. Let's take the average and say 5 wounds are inflicted. Armor saves (AP 3) and cover saves (template weapon) are denied, so no saves are possible. After saves, then that number is doubled to 10 wounds. Because Str 6 is double Toughness 3, each base suffers Instant Death. The entire unit of xenos filth is purged for the Emprah.
How's that then?