I'm GM of a guild and over the course of the evening I have about 20 people online in guild, but Census "forgets" some of them as the night wears on and people log off. It doesn't lose them straight away but it definitely decreases as time passes.
Well, I know say 20 people have been online over the course of an evening and when I check my guild in Census it will say maybe 16 members or something. It also decreases with time so last thing at night it might even say there is only 12 players in the guild.