You should not have to convert the Number to an Amount in order to have retroactive variations carried forward to the current pay.
Try doing a Payroll Simulation with the "Display log" box checked, and look at the differences in processing for WTs 9170 and 917. As for WT 9172 in table DT, I am not familiar with that table.
An alternative would be to use Outflow and Inflow Technical WTs to transfer the variations from the recalculated pay to the current pay.
