Is this related to the transformers and their windings?Hi Ron,
Yes, most likely, as stray capacitance in the transformer, and possibly insulation resistance, figure to generally be key factors in the ac leakage I referred to.
It's interesting to hear your observations. I have also routinely done this for many years, but I haven't taken the time to assess the differences it may have made, my philosophy being that it can't hurt and might help.
One point that's worth noting is that the value of the voltage readings will vary considerably depending on the input impedance of the meter that is used (which is commonly unspecified). Since the resistance of the leakage path in the component will typically be very high (on the order of several megohms or more), a meter having a relatively low input impedance (such as many analog meters, particularly on their lower voltage scales) will load down the voltage and indicate a much lower reading than a quality digital meter that presumably and hopefully has a high input impedance. The higher the readings are that the meter makes possible, the more pronounced the difference between the two voltages will be.
Best regards,
-- Al