I have different computers I use for different purposes. I have been conducting a number of speed tests on a new RBI antenna configuration and have run into something puzzling. One computer, running Win 7, gives consistent results of 30+ mb/s down on both Ookla and nPerf. Another machine, running XP, usually gives 30 mb/s on Ookla but only around 5 mb/s on nPerf. Both computers have the same LAN connection to the gateway device. I can't see anything different about them other than the OS. 

So which results should I believe? Why is there such a difference? NPerf also shows a much higher latency on the XP machine. It is a slower computer and has limited RAM but would that affect speed test results? I find this somewhat confusing.