Let's assume FFG wants the power level of "single lasguns (essentially all S3 equivalents) have a remote chance of damaging Marines in PA". Let's assume they merely overlooked giving a rule such as "RF for all troop-tiers that don't have extra damage dice".
Let's say 50 Guards fire at an Astartes (at whatever range) and 25 hit. Some hit more than once, generating 40 hits in total. Of those, let's say 4 will potentially generate RF. All others will be harmless. 2 will confirm RF. They do each 18.5 damage average each, 2.5 damage coming through each... so a total of 5 damage on average. If one was to roll for all 50 IG individually.
Now if you roll well, maybe 4 score righteous fury and one of them is a double 10. The 3 others score on average 2.5 damage. add to that the 12.5 damage for the especially lucky one and you get 20 damage getting through!
All this gets abstracted away by the extra damage dice. 3d10+3 averages 4.5 against soak 16. But it can go up to 17 damage.
And I like that this is so unpredictable. This makes advancing under fire a risk and if successful, an accomplishment.
Alex