Ex. With-holding Tax : Due Dates

Dear Experts,

In configuration of EWT, I have a mentioned EWT Due date as 777, which means, 7th of every month.

Now when i m trying to run EWT MIS thru T-Code : J1INMIS, i m observing that for the same EWT Tax Code, some the due dates are as 7-Jan-2010, but for a few these dates are something different.

Please help in reasoning, i have researched hard, but not successful at my end.