First, setting cachetime=0 is good idea while testing this. Furthermore, I am thinking of removing caching completely from this extension, because it confuses most people because updates seems to be awkward...
If the time period used for calculating the data is smaller than 5min or you are using "today"-mode and time is before 0:05 a.m. (board default timezone), this can happen, because "who is online" is based on the last 5 min and then could potentialy catch more users in "currently online" than in "who _was_ online" because the period is longer.
However, you probably do not fall into this category during the check, right? Then it would be a bug. But I cannot replicate the bug, yet. :-/ Someone else having the same problem as leschek?