Writing rules for other people is much harder than writing rules for yourself.
Writing anything like instructions for other people is harder than just understanding what needs to be done yourself. I've heard it said, if you want to see if you understand something, try to teach it to someone else.
Gary Gygax realized this, too, with a game he published in the 1960s, Arbela.
"A few months ago it seemed that Arbela had very complete, clear rules...Part of the difficulty arises being too close to the game. I know the rules and therefor do not have to consult them, so it is hard to find any parts overlooked or unclear."
- Gary Gygax, quoted on Playing at the World
This is part of why I say that game writing is technical writing. And even knowing this, it's still possible to write rules that don't quite convey the intent of the writer to the reader of the rules. I've experienced this and I'm sure every game writer has experienced this. And clearly, Gary Gygax experienced it.