[old_tv_nut]

The old motor driven clocks followed the precisely controlled 60 cycle power. They could not respond to transients on the power line.
Later electronic clocks followed the 60 cycle power (many still do), but were (are) subject to counting occasional transient spikes as additional cycles, and therfore run fast.
You may have a clock that depends on an internal reference, not the 60 Hz power frequency. However, three minutes a month is terrible accuracy for a crystal controlled clock,if that's what it is. (Think of a digital wristwatch for comparison.)

I have a little crystal controlled clock radio (runs from batteries or a wall wart DC supply, so no line frequency reference) with much better accuracy than you are seeing.