Actually the framerate difference of 10 half-refreshes goes far beyond the time when microprocessors were used regularly.
NTSC was the first system to be created, at 60Hz with 480 line (640x480) definition.
PAL was developed a bit later, at 50Hz (just a choice, but it may correlate to the fact Europe having 50Hz 230V electricity compared to North America's 60Hz 110V, correct me if this is completely wrong) and a bit higher definition, 576 lines (720x576). It also had bit better colour definition.
In the case of the clock running a bit more slowly, PD was just too lazy to convert the 60Hz game into 50Hz. This also means that the cars, and the whole game run slower.
But TBH I've never noticed the game's timer running slower than the real clocks and I have the PAL version on a PAL console.