Are you testing to the same server? Do you have data (if any) disabled when you test?
Are you running a few tests on each device, alternating devices, and averaging the results for each device? I can get anywhere from 7mbps to 25mbps testing on my laptop to the same server over a 30 minute period. So what's my "speed"? 7? 25? Somewhere in between? Numbers are useless unless you understand what they mean and how they're derived.