Something that has always bothered me about the fluff behind space marine chapters.
It has been said many times, in many sources, that they are 1000 battle brothers in size. Thats the established upper limit according to the codex astartes the bulk of chapters follow.
However, in some of the fluff, it lays out a much different picture.
For one, even a basic company has more than 100 marines.
10 squads of 10 marines and a command section (captain and command squad for 6), plus usually a chaplain.
Now add in the crews of any vehicles attached, so a basic company has at least 9 drivers for the rhinos and razorbacks, plus any attached speeders, again which most companies seem to have a few of in any piece of fluff, so maybe another 6-10 crew.
Now add in any dreadnoughts the company has (and most seem to have at least a couple).

So now a basic company has, low estimate, 120 marines.
Add in the armory a chapter has (predators, land raiders, whirlwinds, vindicators etc) which is probably another conservative 60 odd marines, plus at least another 20-30 techmarines, the chapter librarium, and apothecarion adding another 30 or so on top of that.

Excluding the 10th scout company, your chapter now stands at over 1200 marines as a bare minimum.
A complete "order of battle" for the Ultramarines is given in one of their codexes, and after going through and adding it up, they are over 1300 marines, and several companies are "understrength".

So I ask, as a discussion, is the established value of 1000 *just* fully-fledged battle brothers, excluding all vehicle crews and specialists and everything else, or am I missing something?

This would mean rather than approx. 1000 chapters of around 1000 marines for a total of roughly 1,000,000 space marines in the Imperium, you're looking at nearly a quarter million extra marines, or over 200 extra Chapters. Doesn't this seem like something someone would have caught on to?

Yah, I would not count individuals, support, and leadership staff. I bet they didn't even think of marine transport drivers, so just counting Tactical, Assault, Devastator and Veteran Marines, Hopefully Bikes too. not scouts or Tech/Libby/Chap.

Add to this interpretation of 'Battle Brother', the prevalence of chapters deviating from The Codex, there is a fair amount of room.

I've actually noticed this oddity too, but didn't think much of it, though fluff ignoring things like this tend to. You also have to remember that space marines still die, and new ones still become fully fledged Battle Bros, so there will be a constant (if small) fluxuation going up and down from 1,000. THat still doesn't really exdplain it, but it adds uncertainty, which leaves room for a couple hundred marines prior to a campaign, and just under 1,000 after one.

Yep I think along the same lines.

1. Its counting only main line marines (Tac/Assault/Devs/Vets)
2. There is constant flux in the actual count.