Performance Seat Mail Report Question

Hello! We're in the midst of our subscription period, with some series and events currently in various stages of seating and payment. Our ticket office manager suggested the Performance Seat Mail Report (odd name -- any explanation?) as a way to show all tickets assigned to a performance/price type, even if they haven't been seated.

But when I total up the sales on that report, it doesn't match the total money that has come in to date -- we're off by 3-4%.  Not huge, but we can't figure out the discrepancy.  For total $$ in to date, we're looking at the cash + amount committed through installment billing. 

The help doc for this report isn't particularly helpful, so I'm wondering if anyone else out there has any explanation of what's included in the Performance Seat Mail Report and how you use it.

Thanks!