It's less a roll of the dice and more that cover bonuses are applied by the entity, but the 'in cover' symbol only indicates the squad. It is either when the squad leader is in cover, or the majority of the entities are in cover, that the symbol appears.
Either way, there's often a high chance there are entities NOT in cover that are eating all the suppression for the whole squad.
Cover is applied by entity and suppression applied to the squad. This is what causes that sense of 'dice rolling'.
Infantry just are too stupid/blind in combat to move or position themselves out of open cover, or simply can't find their way to cover on their own.
There are other factors as well. There's something called incremental suppression which in theory is to help with suppressing blobs. What happens is that this doesn't apply on number of squads rather than models been caught in the search radius. Unless something has been changed, this means that it's easier to suppress a conscript squad rather than Rifle or IS. From the little test i did a while ago, it seems it applies per 3 entities caught in it's search radius (i had discrepancy between Rifles and conscript getting suppressed behind green cover).
The thing is, if you have +12 models, you will get suppressed fairly quickly by a MG42 (4/6 burst) even if you are perfectly behind green cover at medium to long range.
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