@pp4mnklinux :-
It's probably because of the way the entire infrastructure has been set-up.
As you quite rightly say, Chromium is the parent.....not only of Chrome, but also Opera, Vivaldi, Iron, Slimjet, Maxthon, Yandex browser, even M$ Edge. Etc, etc, etc....
Chrome & The Chromium Project were set up simultaneously in mid-2008. The intention was always for the browser code to be open-source, and indeed, that source code is contributed to by thousands of developers worldwide.
The Chromium Project is seen by many as Google's browser R&D department. It's where ALL the cutting-edge development work takes place - making use of the newest versions of absolutely everything - and the Project has an entire fleet of semi-autonomous "bots" that do nothing BUT churn out build after build of the browser, 24/7, as & when 'patches' and/or 'fixes' are 'slipstreamed' into the automatic build-system by the contributory developer 'ticket-system'. The system IS subject to regular human oversight, however. Just because a bot may consider a particular build to be more stable than others, it doesn't follow that those responsible for monitoring output will necessarily agree...
The Project is basically bank-rolled by Google in any case. As & when what is considered to be a stable build is created, approximately every 4-6 weeks, the responsible Chrome devs will take the code for that build, add certain Google-specific proprietary code, re-compile it under a much older build-environment than that specified, re-badge it as Chrome and release it as the latest stable Chrome version. The reason for the older build environment is simple; because by doing so, it makes the browser available to the widest possible user-base.......since too many folks, IF left to their own devices, would never bother with system updates at all!
Since both Chromium AND Chrome are essentially the same browser - from the same people - it probably makes very little sense to write two completely separate sets of help documentation, when what applies to one directly applies to the other.
From an original 'beta-tester' all the way back in 2008, that's MY two-penn'orth.....FWIW.
Mike. 
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