Why: help ignore: set number in .vimrc?

I :set numberin ~/.vimrcto show line numbers. When I print :helpto view documentation, line numbers do not appear in a new window.

If I type :setl number?, it prints nonumber. If type :setg number?, it prints number.

I want to know why it ~/.vimrcdoesn’t work. Which script resets the local option number? I checked $VIMRUNTIME/ftplugin/help.vim, but not reset number.

I am currently using:

if has('autocmd')
    auto FileType help set number
endif

Thank.