There are excuses that are perfectly reasonable to go over by 1 mph. Speedometers are not perfect, and neither are radar detectors. All my cop friends have told me they are trained to give 10% of the speed to normal variation, so at 50mph you would be within the limit to go 55. This has been my personal experience while driving too.

All of science is just a sample. Population trends can be observed in smaller subsets.