Contract Billing in SWM

Hi,

I have an issue with contract billing as follows:

A 2 line contract was created in F1721. line# .001 was for 1 year & line# .002 was for 2 years. The first line was for Start Date 10/01/2013 & End Date for 09/30/2014. Billing from date 09/28/2013 to date 09/26/2014. F1721.DOM = 1 & Billing frequency =5 i.e. Half Yearly May & November. Amount was in HKD 13758. So the recurring billing amount is 6879 HKD. Now on 11/01/2013 the first billing was created as expected but the amount is 7937.31 in place of 6879 HKD. I did all kind of calculations but somehow this figure does not match anyway. I want to