Same setup different hash rate I have one computer with windows 8.1 13.11 driver and a 280x which gets 720KHS and another computer with the same setup which gets 620KHS.

They have a different cpu ram and psu (both have quality ones).

How can this be?
I have one computer with windows 8.1 13.11 driver and a 280x which
  gets 720KHS and another computer with the same setup which gets
  620KHS.
  
  They have a different cpu ram and psu (both have quality ones).
  
  How can this be?


It's because they have a different ram. With scrypt mining RAM is of up most importance. The faster/higher quality the ram, the faster you can mine.

Scrypt was made to require ram so it would slow down the over all hashing power and to resist asics.