I'm confused as to inclusion of wages in computation of NIIT and would appreciate clarification. I have read statements which explicitly say that Wages are not included for NIIT computation. But if you look at instructions for Form 8960 line 13, AGI and MAGI do include Wages. MAGI is used to compute NIIT. So what is really true?

Secondly, assuming wages are included, why? 3.8% has already been paid by the employer and employee. Why does 3.8% need to be paid again as NIIT?

Obviously, I'm missing something and I'd appreciate help in putting me straight.

Thank you.