In the final pay run of the tax year, why is my Director's NIC amount different compared to last month?

The rules for calculating NICs for Directors are different to employees. Director’s NICs are calculated on an annual earnings basis. When using the 'Director as Employee' setting in our software NI is calculated for the Director through the year just like a regular employee, and then the annual calculation method is applied in the final payment of the tax year i.e. Directors rules are ignored until the last period of the year when a NIC adjustment is made, either a refund or extra payment.