They are using this formula here

http://en.wikipedia.org/wi

If I plug in the numbers there except they use t1 = 0 for the left side and t1=1 for the right side

Left side of the equation is just total loan amount

10000 = SUM(k=1 to M) of (P_k*(1+APR/100)^(-T_k)

Where M is number of payments, P_k is payment number k, and T_k is the time that has passed since the loan started (in years).

If I plug in 11.457 for the ARP, the equation works out perfectly if I use T_1 = 1/12, T_2 = 2/13 etc.

It also works for any other combination I tried.

The Wikipedia article says T_1=0 though.