Calculated credit for taxes paid to other jurisdiction (Philadelphia) on NJ tax return is always less than actual tax paid.

I am doing my taxes with TurboTax every year and always get the credit for taxes paid to Philadelphia. However, the amount of credit applied (line 9A in the form NJ-COJ) is always less than actual amount paid to the city as stated on W-2. This year it is about  22% less than actual amount paid. How is the credit calculated and why would it not be the full amount paid?