Just an idea.
These type of watches usually generates its internal reference clock from the main supply frequency. Possibly you are supplying it from a 60Hz main supply instead of a 50Hz main supply.
In this case I expect a 60/50=1.2 ==> 20% faster running, but since the dividers are designed to generate an integer number of pulses with 50 Hz, using a 60 Hz the non integer part of the division is lost, slowering a bit.
But as I said, is just an idea.
Sorry Barry, I was writing when you posted. I just realized I've posted the same idea you posted