Don't use your Satnav to judge your speed, they're too unreliable.
How do you know that your speedo wasn't at 89mph at the time you were driving 89mph?
I've always 'heard' that you can drive 10% over any limit without ever flashed.
It looks to be the case.
The brackets are intended for those drivers who may be unaware that the speedometer has crept up and allows driver to attend the course to better make them aware of their speed...