I get to see the lot and the guest count tends to go higher as the member count drops so it appears to be the case that not all members clock on when they are reading the forum. The other factor is the number of people connecting to the forum is seasonal, less in the summer, more in the winter, its been this way for years.
I am on the other side of the world to many members so I regularly only see me online when its middle of the night on the other side of the world. I see most people online when its the middle of the night here in OZ.