Consolidated bill invoice amount does not match with attached invoices

We have come across scenarios in Fin 9.0, where Consolidated bill invoice amount does not match with attached invoices. For example, consolidated 831047952-2 (19,950.00 before tax) with 831047820 (-19,950.00 before tax). So I was excepting the amount to be zero, but for some reason the total amount was 5187. Any idea as to what could be the reason. Both the bills are of same currency.