The last time this was explained in full detail was waaay back during 2nd edition, though. I have a feeling it hasn't been described in more recent times as it makes the Astartes feel less important than the Guard, which is a notion the studio may wish to avoid, even if it makes perfect sense due to the Marines' relative rarity.
I've run into this with my mass combat system a lot. GW consistently paints space marines as the foremost of the Imperium's armies and a greater part of the fanbase share that view. But it's unfortunately completely irreconcilable with the idea of deploying only one Codex-compliant chapter to a battlefield. Even allowing 100-marine companies to fight on the same scale (but with decisively superior stats) as 10,000-trooper IG divisions, the fact that there are only 10 of them means that their only hope of victory against almost anyone is to deepstrike directly on top of enemy reinforcement points early on and hope to clear them out before they can bring a numerical advantage to bear. Ironically enough, the enemies marines are most famous for fighting (orks, Chaos, and tyranids) are also the ones they suck the most at fighting, since they all have alternative means of getting reinforcements. Especially orks.