No body is driving perfect, speedometers aren't perfect, and such automatic devices aren't perfect nor can they identify the driver yet. I believe they use these elsewhere like UK... But most likely have a bit of wiggle room
It may be necessary to speed up to avoid or pass something reasonably and ultimately the speed limit is so for most conditions. In a 70 it may be reasonable to do 80 on a clear, sunny day with minimal traffic. After all it's still 70 at dark when your visibility is cut way back. Plus it can't be a hard rule given weather conditions, traffic speed differentials, etc... 70 may be too fast or otherwise.
It's not that big of a deal unless you're the one driving too fast or slow relative the everyone else.