To give an example of how this could happen (not saying this is what happened, but I've heard this happened before):
Suppose you left ntpd running and automatically adjusting the clock every hour.
If your clock is running faster than pool.ntp.org, and you are synchronizing to it, you may end up adjusting in the middle of an event. Because your clock is running fast, you would jump back in time, breaking the sequence of time (this is somewhat equivalent to what you see during daylight savings time if you aren't intelligent in the way you handle the backwards hour shift)
In this case, if the adjustment was forward in time, there would be a gap.
Suppose you left ntpd running and automatically adjusting the clock every hour.
If your clock is running faster than pool.ntp.org, and you are synchronizing to it, you may end up adjusting in the middle of an event. Because your clock is running fast, you would jump back in time, breaking the sequence of time (this is somewhat equivalent to what you see during daylight savings time if you aren't intelligent in the way you handle the backwards hour shift)
In this case, if the adjustment was forward in time, there would be a gap.