I think the problem is that you have marketing people and practical people.
Marketing people want the manual to look great but assume that people don't read it. The manual is just filled with random text designed to sell the game.
Hence, the marketing people must make decisions like how wordy a manual should appear for the desired audience. If the game is for hard core people who like a story, the manual should be very thick and cryptic. It should be filled with tons of amature fiction which the marketing people have scoured off the internet. This fiction should be modified to substitute game ideas for the original terms but otherwise be left in tact. If the game is for light hearted gamers who just want a hack and slash, it should be relatively thin and consist mainly of illustrations and have little if any text content.
A manual is just a selling tool and is meant to create an impression. If the manual is cool looking, maybe the game is cool too..
Practical people on the other hand see the manual as part of the game. Any content in the manual is meant to help improve their gaming experience and the manual is meant to be read. It is not meant to deliver technical details which the author may not care for (readmes are writen so game devs can boast these). It is purely meant to show a player how to get the most out of the game. It may contain backstory but it should not be dull or boring. It should be written like people will read it with good artistic style and wit.