I have noticed differences myself, but don't pay much attention to them.
As far as I am concerned there are a lot of possible reasons. It may not take into account the upkeep cost for units that are about to be created...if it applies. The most obvious I can think of is farming. It must use a given havest (good harvest?) number when calculating the projected treasurey total, but in reality the harvest output is often lower.
[This message has been edited by Themistocles472 (edited 10-17-2005 @ 01:43 PM).]