How can you explain this additional amount?

An employee receives a pay statement with an additional amount. This amount is classified as a retro calculation difference .

How can you explain this additional amount? There are 2 correct answers to this question.
A . The employee was hired, but the hiring date was effective in the previous payroll period.
B. The employee entered time on the timesheet and changed one of the future dates.
C. The employee received a promotion to another department, which was effective two weeks earlier.
D. The employee left the company a few years ago and returned to the company as a rehire.

Answer: A,C