Spatial Reference System

Hi, I'm using DJI Phantom 4 RTK fixed photo geotagged and processed 3mx. The result is questionable in height, approx. 2m different with our GCPs.
While checking the result of GCP location, I found different value in same reference system. I upload my screenshot for your perusal.

WGS84Height: 30.373m
WGS84 EGM96 Height: 32.744m
4393+ EGM96 Height: 30.557m
4393 Height: 28.192m

Our GCP Geiod Height: 32.391m
GCP Ellipsoid Height: 30.127m

My question is, why EGM96 height different value when i use default WGS84+EGM96 and my 4393 override height EGM96?