You're always gonna have different results because of the dependency on local and remote bandwidth. Your hub/switch/CMTS/DSLAM could be heavily used, while the correspond unit for the host server of the speed test might not, or vice-versa, or both.

At 3am you may have your peak speed results, but at 5:50-9:00pm you might have the slowest results - all due to traffic near and far.