Hello,
The manual says that if you press the dollar icon it will contain, among other things, the amount that your taxes will go up or down next turn. It does not.
I am in a game where I have 800 total taxes and trade and 200 military upkeep. So should my taxes not increase 600 next turn? They do not, they increase only about 130. Can anyone tell me what is going on? I am a new player but all the other aspects of the game are understandable. Any help appreciated.

Noticed this too. There are other misc expenses such as the automatic transport of resources from colony to colony each turn. But these cannot account for all this mystery shortfall!

You are using the money to transport stuff or on maintenance costs for your military units.