Chances are that if you check our calculation against IRR or XIRR there will be a difference. Our calculation uses a full discounting method, which necessitates an iterative calculation to solve it. The method that we use is standard in calculating yields in bond and loan markets.
IRR does not produce an accurate calculation where the payments are not exactly regular, and XIRR assumes discounting on exact dates, rather than periodically. If you would like more information on the calculations please contact us and we will be happy to help.
Please do contact us at firstname.lastname@example.org for any further assistance or information you may require in this regard.