When you run a Tax report you can choose to run a Cash (based on when the client paid), or an Accrual (based on when the event was that they paid for). This can affect which projects generate in these reports.

For example, imagine I paid you 400.00 in July for a project happening in August.
I'd see this project when running a Cash report for the month of July.
The Accrual report would only show the project when viewing August.