Leave amount is based on the hourly rate of the position history that was active at the time when the leave was booked.
December 2019 - Salary: €12,000 (hourly rate: €5.77)
January 2020 - Salary €15,000 (hourly rate: €7.21)
Therefore if leave from December is added to the January payslip, the Leave Amount will be calculated accordingly. If you then had to divide the Leave Hours by the Leave Amount, the hourly rate will be an average of the above two position histories.
The above also applies to the SSC Class calculation. Where the SSC will be lower in the above case.