Bill that was raised under a contract did not cover the full negotiated amount

HI ,

we created a contract and the bill plan was negotiated for 200000.But the amount that was billed was only 190000. Any idea why it happened like this?

In the Define Billing plan screen, under Billing plan General tab, in the Amount detail section,  the Billing amount shows 200000 but Total Billing history is showing only 190000.

Where I have to look for the solution so that it will not happen again.

Thanks in Advance.

Sudarsan