European cars tend to exaggerate speed slightly. Up until the moment you learned that, you thought you were an undefeated speed demon.
Tire pressures and rim size have a negligible effect on speedo readout. The engineers build this cushion in so that even if there are variables out of your control, there's no way you can sue the company for an inaccurate speedometer.
Make sure when swapping for winter that you buy rims and tires with a total rolling radius equal to factory spec.