shell_manager.cp doesn't copy if the source file has changed

[Waxie]

Another question about cp:

If I have the command

cp("/source/dir/.", "/dest/dir")

It will copy all files in /source/dir to /dest/dir that don't already
exist there. However, it won't re-copy any files even if the source
file has changed. It does re-copy if the source refers to a file
instead of a directory.

Is there something fundamental that I'm missing about copying a
directory full of files? Should I be approaching this a different way?