Total Sales Report doesn't match Total Sales by Product Report

[wgayda]

I'm having an issue with a client's shop report. I'm trying to split up total sales over the past month by product, but the "Total Sales by Product" amount does not match "Total Sales" number on any other report. Does anyone know what might be causing this discrepancy? Are there somehow sales that aren't showing up on the Total Sales by Product report that I need to factor in?