Eric may have an idea?
In no particular order, these things occur as possibilities:
1. A bug in DSLstats. Always possible of course, but as this issue only affects a small proportion of users I don't think it very likely. I've pored over the relevant code many times, and it looks robust.
2. An inaccurate PC clock. If the PC is properly synchronised to an internet time source, this can be discounted.
3. A second instance of DSLstats running on a different machine. This wouldn't happen by accident, so I think we can discount it.
4. Inconsistency in the time taken to collect samples. If the PC is connected by WiFi I would rate this as the highest possibility. But it's still possible with a wired connection, depending on the modem and the PC and how heavily loaded they are.