Most Online Counter Wrong Results?

I run a fairly decent sized forum 56k+ members with an average of 500+ people on at any given time. However, occasionally the "most online" statistic will jump up by a few thousand. For instance today it said it was 6k when before the highest was around 2500, looking at Google Analytics don't see a huge increase in page views, nor a big spike in new registrations. Most of the visitors were guests. Is there any explanation for this? I did notice a higher than normal server load.