Maybe you could find a way to get the Census Addon to get a dump of the entire guild (online and offline) when it runs. My guild has 318 toons and the census her still shows over 500. An error that large could seriously skew some numbers.
Right now a person is listed as part of a guild until they are polled online not in a guild or in anoter guild.
For instance, an update by a person in guild WiccanWay contains a guild dump and it shows a previous member no longer in the guild, that missing toon could be moved to an uncounted section until they are Census'ed either as upguilded or in another guild.
This would help eliminate those people who just stopped playing or switched realms and make the data a lot more accurate. Especially in guilds like mine that flush dead toons after 3 months of inactivity.