"add to selection" is rather "invert selection"
Take e.g. a multi-voice piece, select (a) note(s) in 2nd voice -> right click -> select -> more... -> add to selection
Instead of adding to it inverts the selection, i.e. the note(s) you started with is not part of the section
This is true for 1.2 and trunk (GIT commit: 6dbce9b)
In turn 'replace selection' really does 'add to selection', so at least "add to selection" should get renamed to "invert selection"
Possible 'replace selection' could get renamed to 'add to selection'
This is not necesarily a change in the code, it could be done in the translations alone, but that would be pretty confusing (to the translators).