Common reasons why this could occur:
1) Previous pay and tax data from the employee's P45 - check to see that you have entered these correctly. If you have then perhaps the tax was calculated incorrectly by the previous employer. The package refers to figures entered from the P45 to calculate the current tax due. If the employee previously paid too little tax, the program will adjust the tax payable in the current pay period accordingly.
2) The employee's gross pay entered in the current pay period is too high.
3) The employee is now on a Week 1 or Month 1 tax code which means that tax will be calculated as if it is the first period of the tax year.