It is estimated that each electron pair donated by NADH leads to the synthesis of approximately three ATP molecules, whereas each electron pair donated by FADH 2 leads to the synthesis of approximately two ATP molecules What is the underlying reason for the difference in yield for electrons donated...

The underlying reason for the difference in ATP yield for electrons donated by FADH ...

It is estimated that each electron pair donated by NADH leads to the synthesis of approximately three

Question:

It is estimated that each electron pair donated by NADH leads to the synthesis of approximately three ATP molecules, whereas each electron pair donated by FADH₂ leads to the synthesis of approximately two ATP molecules. What is the underlying reason for the difference in yield for electrons donated by FADH₂ versus NADH?