Why are Leavers not seeing equal calculations of BIK when terminated?

We have noticed that when a leaver is processed in the system over a month before they are due to leave (so they have 2 or more payroll runs left) the taxable benefits for cars being affected. The system seems to take a smaller amount in month one and two and then the remaining amount in their final pay, which is a larger amount.

The total amount of BIK, by the time the employee leaves, is correct, but we cannot understand why it does not calculate equal amounts in the remaining months. It's causing a lot of questions and disgruntled employees when leaving.