@Sebastian_Angga, I agree with your math. It seems to me that what is happening is that Manager has correctly calculated the total late fee for the first month the invoice is overdue. But for the second month, it appears to have multiplied the new late fee (which should be $1,100) by the number of months late to derive the total late fee of $2,200. What it should have done is add the new late fee to the old total, including the first month’s late fee.
I can only guess what it would do for the third month. I suspect it would take 10% of $12,200 and multiply that by 3, in other words: $1,220 x 3 = $3,660 for a total balance due of $13,660. Of course, the actual amount due at that point should be $10,000 x 1.1 x 1.1 x 1.1 = $13,310. The formula should be TOTAL x (1 + INT) ^M, where TOTAL is the original sales invoice total, INT is the decimal value of the penalty rate, and M is the number of months late.
Now remember, I am only speculating on how things were calculated. I don’t know, because I’m just a user like you. But there does seem to be a bug that @lubos should check out.