The definition of elevation

Hi everyone,

I am calculating the LOS displacement and trying to compare with the GPS data recently, and I read that the elevation is with respect to a reference ellipsoid(WGS84) in the InSAR Principles Part B-42.
But when I compared with the GPS data, I found that some points have huge differences between InSAR data and GPS data.

So first, I want to make sure that the elevation is ellipsoidal height, not orthometric height.
Second, Is it possible that the difference between elevations of InSAR data and GPS data is almost 100 meters??

And BTW, the DEM I used is SRTM3.

Really appreciate for your help!!