The manuals are written when the machine was built, or as they are built, then once those machines are in the field being used and needing repairs. With the modern age technology of computers and cell phones, most techs can access anything they need while right in the field, directly to the manufacturer and get an answer back almost immediately when the manual is either wrong, or incomplete. Once a mistake is noted in the manual, that update is given to those answering the questions in tech support, not updating the manuals. So after a dozen years later and once out of warranty, and the rest of us idiots buy these machines and try to fix them and work on them ourselves, or work on a different brand of machine we don't have a franchise for, tech support is almost nonexistent and the manuals are so lacking its not funny. Very poor manuals are another way to require OEM to continue to work on the machines they sold. Now this all depends on the manufacturer, some are very good at providing tech support, others are so bad, to say they suck is an understatement and every area of the country and world is different. As they say color sells many machines, the support and service sells far more, and that's dealership dependent. So when some brands sell great in one area and nobody will even consider that brand in another, it gets down to the service side of the equation, and yes service manuals are part of that equation.
Lately [I've done it many times] I ask to see a service manual for a machine I'm considering buying, if nobody can provide one for me to even look at and read, chances are I'm not going to buy that machine no matter how good of deal they intend to give me. Thats because I'm going to be the one who ends up fixing it myself at some point down the road and I want to know, when I do buy a used/new machine I can also buy a descent service manual that does me some good. I've pretty much weeded out the manufacturers who manuals suck and tech support is nonexistent, you know, being told to just call the local dealers and have them fix it, constitutes tech supports that sucks in my opinion, but since that changes from year to year, I also need to know who's getting better and who's getting worse.