Since this is a Payroll related inquiry, it would have been more appropriate to use the Payroll Discussion Forum (SAP ERP HCM Payroll) and to specify for what Country.
Next, look at your IT0008 Wage Types configuration in table T511, and mention how they are set-up.
Then, do a Payroll Simulation for that employee, with the "Display log" box checked, and drill down into your schema to look at your Partial Period Parameters and Factoring. Note that there are many Discussion threads on the subject.