Flashed at 62 in the city – Measurement accuracy?

Hello, something strange happened to me…. I drove with Tacho 55 through the city, when it suddenly made Ping! Great… I was wondering why they flashed with such a small overrun… Now the letter came and it says I should have driven 62 kmh… After deduction of the tolerance it should still be 9 too much = 15 € As already written my Tacho showed me quite exactly 55 kmh. The Tacho goes very exactly with me. With Tacho 100 it shows me only 2 kmh too little on the GPS. Wa r maybe the speed camera set wrong?? I don’t see it one to pay for something like that! It’s not about the 15 € here, but about the principle. PS: Speed camera was out of the city from a car. Greeting Patrick