Acer 5101 - Why are there screws missing from my fan?

I have just opened up my 5101 for the first time, to clean out the fan after the processor started throttling down while playing oblivion, resulting in fps drops (man, did it need it, Im surprised it hadn't melted yet). When I was opening it I noticed one of the screwholes on the outer cover had no screw in it and on opening up the cover I found that the fan is held in by only two screws on the top side (next to the back vent) and there are none on the other side, although there are holes for two more. Is this deliberate? Should it only be held on by two screws? or has some muppet in the acer factory been saving money or cutting corners by missing out screws?

When I put it back together should I put some screws from the cover into the fan? Could this be causing problems - It doesn't seem overly noisy or anything.

Thanks in advance

 

Nevermind, It seems the missing case screw is the same one as for the fan at the bottom, i just replaced it with a different one from the panel.

 

I see a lot of 5100's with missing screws... it seems that they just fall out after a long period of time.