I think it might have to do with the LAN ports on each PC...sounds like one is a 10/100 and the other is 10/100/1000 (gig port). Depending on the time of day your checking your speeds, how much traffic is on the net when your checking...can affect your speeds.
Looking at our mobo specs, I see it has a 10/100 LAN port....you could replace that with 10/100/1000 PCI card. That will definitely help out with your data speeds. ASRock > Wolfdale1333-D667