On your PC, with one user, what you say is true.
On a Debian build-daemons or the GCC compile farm, for example, the
system headers change all the time, as developers upload new, modified
versions of all the libraries, some of which fix bad defines in header
files.
In your personal case, it is rare, yes, but in a wider context this
would give broken builds where the system headers have been fixed, or
apparently successful builds when some turkey actually just broke your
app by fixing the system headers.
