R11i : question about the invoice due date calculated from payment term

Dear Sirs,
I have a question about the invoice due date calculated from payment term in R11i

I applied the patch 9226088 (Doc ID 1552657.1) in internal instance.
As a result,the invoice due date gets as followings;

  the Payment Term is Setup as :
 
  Last day of month is flagged
  Day of Month : 15
  Months Ahead : 2
 
 Enter transaction trx date 11-Dec-2009 with this payment term -> Due Date is 15-Feb-2010.
(This is same behavior for Manual invoice and form autoinvoice)

But I have difficulty to understand why due date must be 15-Feb-2010.

If Trx date is 11-Dec-2009, the last day of the month is 31-Dec-2009.