Box 5(medicare wages) doesn't sum up to box 1 + 401k contribution (box12 D)

Hi,

 

I am looking at my W2, and my medicare wages(box 5) is less than the sum of my wages & 401k contributions. Note: I didn't use any medicare. 

 

Box 1: $x

Box 5: $y

Box 12(D): $z

 

In my case, $y is less than ($x + $z) . I suppose medicare tax withheld is also lower than what it should be. 

In other words, I am trying to figure out the exact differences between taxable medicare wages in box 5 vs taxable wages from box 1.  

 

One delta is 401k contributions, as "taxable medicare wages" is supposed to include this amount. 

 

What should I do? 

 

Thanks!