Originally Posted by
JamesP
I suspect there's been a design decision at some stage to sculpt vehicle crew with the Opus Machina etc and the background has taken a different direction.
I think that depictions of the number of Marines in a Chapter always have to be taken with a pinch of salt. For example, the Chapter breakdowns that I've seen have the apparent problem of insufficient Techmarines to provide vehicle crews, as you've noticed, but also doesn't account for Marines commanding fleet vessels (say at least 1 per escort class, more for Strike Cruisers and again more for Battle Barges), on detached duty with Deathwatch or even overseeing their fortress-monastery's defences, though the latter could be done by whichever company was currently stationed there.