Posting run for employee terminated in the past pay period

Please halp me to understand how the system decides if the employee has to be selected in the posting run.

There are two examples:

Severance payment has been created (via IT15) in the current pay period for the employee that was terminated three pay periods ago.

Now, if the posting is run for the old pay period then the system selects and evaluates the employee (though cannot create posting documents as the pay period is closed), but if it is run for the current pay period then the employee is not selected and not evaluated.

On the other hand, another employee terminated starting from the day in the past pay period (the termination action was run in in the current pay period), is selected and evaluated when the posting is run for the current pay period.

Thanks a lot!