So for the people confused, the screws are to hold the fenders in place, not to hold the fenders to the bike.

The 'arches' that allow the fender to move around, can cause them to move after hitting a bump, so if you just put a screw there, you can hold the fender to the 'arch' holding the fender in place so the fender doesn't move on the grooves.

Edit: Botto, the clearances on a 9 are pretty tight.