What's overriding my .vimrc tabstop size setting for CMake files?

In my ~/.vim/vimrc, I have:

set shiftwidth=4
set tabstop=4

and when I edit a plain text file, that is respected. However, when I edit a CMake file - these both becomes 8. I tried looking for the cause of this change, but couldn't figure it out: * Nothing in /usr/share/vim/vim90/syntax/cmake.vim seems to be related to tab stops, or to include another vim file. * Same for /usr/share/vim/vim90/syntax/cmake.vim. * The CMakeLists.txt files I'm editing have no modeline. What's causing this? Is there a definition I've missed? And - can I override it somehow?