gmoverid in dc operating point is not equal to gm/id

Hi,

I just want to do the gm/id plot, by following circuit

.

and i included one scs file with "save NM0:all", i can got gm, id and gmoverid by dc simulation. use following 3 expression to get the gm/id plot.

found in weak inversion region,  why NM0:gmoverid is not same as getdata("NM0:gm"?result"dc") / getdata("NM0:id"?result"dc") ? thanks.