With my Stromer, I can go above 52 km/h. Not downhill, in the plain and without wind. Gear settings are 52 / 11, this means 1rpm = approx. 10m. With a ST1, I couldn't exceed 48 km/h - even with 4 Bar, fully charged battery an all the strength I have. The reason might be the efficiency of the ST1 motor. Above 45 km/h, most electrical power is transformed into heat.
So, maybe you've met a classic Stromer. Easy to identify by the color of its motor.