Well I believe this confuses about everyone. Most people in the business are confused, they tend to use charts and none of them agree.
There are two sets of taxes you are paying, this year's and next years.
The 1 st installment of taxes is due Dec 10. So you pay the taxes from January until June. They only have two months to collect 6 months worth of taxes.
They like to have 2 months of taxes worth of taxes left in escrow after each payment so you actually need 8 months of taxes collected by Dec 10.
Therefore they need to collect 6 months worth of taxes. If they only collected 1 month of extra tax money and the payment is not received by the 10th the account would go to zero of be overdrawn. Remember these are bank accounts with the same rules as bank accounts.
Your pain does not stop there. The seller have already paid the taxes from Sept to Dec.. You will also need to reimburse that money.