What means this Vue devtools version hint?

My Vue devtools show this version hint:

I don't understand from where the version number 3.2.28 is coming though or to what exactly it refers to - my vue version is 3.2.25.

Can't see much more than what you provided in the image, but I think a reasonable guess is that you are using something like this:

  "dependencies": {
    "vue": "^3.2.25"
  },

If that's the case, the install script will get the latest patch version, which would be 3.2.28 at the time of install, and 3.2.29 at the time of screenshot. This will also update the npm (or yarn) package which will make it re-install 3.2.28 even when 3.2.29

if you want to lock it, remove the ^ character, if you want to understand better why and how this is happening look into the Using semantic versioning to specify update types your package can accept documentation