I noticed that. Search engine spiders are supposed to be filtered out, but I'm betting that was an 'unknown' bot. If it wasn't there would have been more visits.
NOTE: I changed the statistics program the first of January (at almost midnight on New Years Eve) - Well, same program but an upgrade. The prior program version did not filter search bots out of the results. This new one does. For that reason it 'looks' like there were fewer visitors in January 2005 than in December 2004 but that probably is not the case.