NavList:

I have used a large number of GPS units over the years, and I have observed on several of them, that the seconds display does not roll over accurately or consistantly.  At times the display will change numbers each second, but other times it will change numbers twice in a second, and at other times, it will change after a second has passed.  I know this because I was observing the GPS at the same time as listening to the radio time tick.  My take on this is that the software and/or processor for these particular GPS units causes the change of the display of the time to be delayed with different processor loads, and then play catch up.  

I don't know if this is what is happening in this case, but because of my experience, I don't use the GPS time display as a standard to set my wrist watch or determine chronometer error.  I ALWAYS use a radio time tick for such work.

Jeremy