Thanks for the report Strider. I will do some research on this issue.
I have noticed the same thing on my Edge 810.
The time gap will suddenly jump by quite a few seconds. I put it down to a poor GPS signal.
I don't agree that the unit is trying to do lots of concurrent calculations for the various lead times, (KOM, PB, other athletes) because when you switch the target time mid-ride, the unit has to recalculate the +/- difference and does not show the gap immediately.