Yea it sounds odd, but the fabric is actually very thin but should be glued in place. Also why I suggested the drill for a pilot hole, makes a hole in the fabric easier. The fabric will help add as a gasket once the sub is torqued down so don't worry about it, there is a reason 99% of subs have 6-10 screw holes, they want you to use them all. Some woofers even come with a thin foam seal on the bottom to make it air tight.

just don't over tighten the screws and strip them out and you will be good to go.