Penalty 38.5 euros, at 7 kmh too fast!??

Hello, I have a question about driving too fast. in summer 2012 I drove 13 kmh too fast and got a warning and had to pay 25 euro. Today such a nice yellow letter with a fine notice of 38.5 euro ( 15 euro fine, 20 euro fees and 3.5 euro administration) is trickling me. because I was 7 kmh too fast. how can it be that the fees are more expensive than the penalty? why do I get a fine at 7kmh and a warning at 13 kmh? why ha b I don’t pay fees at 13 kmh too fast and at 7 kmh equals so much? Feel a bit pulled over the table from the city… I hope you can help me. Thank you