VAT on Sales report

Hi, I'm new to Zen Cart (running 1.3.8a) and everything has run fairly smoothly thus far. After 1 month of being online with the shop I'm trying to compile sales reports for our various manufacturers.
On my invoices, the base price and the price including VAT (19% here in Germany) is all correctly listed and calculated but when I want to compile a sales report (using Sales report 2.0) the VAT is always wrong. Our base price (we sell t shirts) is €15.13 and with the VAT the total is €18.00. The sales reports, however, consistently shows gross price of €18.13 and I have no idea where this extra €.13 is coming from.
Any help from anyone will be greatly appreciated!!