thats pretty obvious to me, its incase you put too big a screw in there, or someone puts a washer under the screw that is the wrong size and it drifts to the side. how many times do you lose washers and end up with something just a tad too big that you are sure will sit correctly?
I think that's a good design decision. Alot of better boards I see use washers near the screw hole, perhaps there was one originally there and it got taken out with a design change or something. And those kits always get mixed up, with a screw it wont fit, with a washer you need to be on top of your assembly game to catch it. Some board designers are really anal about tolerances.
I recently dropped a washer somewhere for a card and then I got a almost identical replacement for it, but when I did the 'can it short something to chassis test", I ended up on the floor for 20 minutes looking for the original washer. Pretty infuriating but it definitely could have caused a problem.