It's also likely the holes have filled up with any sort of dust and muck. I used a long pointy thing to scrape out as much as I could to get to the screws.

In both of the ones I've disassembled, I've ended up being unable to get one of the screws out as the head had rusted to the point of no return and ended up having to cut straight into the side of the shredder to get the top off.