Differences in SRTM data

[s.savastano]

Thank you for your quick reply, as always.
No, also using a different resampling method there is again the same problem.
Reading in the forum, I have found this your post

Vertical Datum of a Sentinel-1 derived DEM

Hi, everyone. When a DEM is produced by the used of Sentinel-1 data, what its vertical reference system? I understand that to transform the relative height to absolute height, it is used a technique called DEM-Matching. If the auxiliary dem used is SRTM, the resulting vertical system of our Sentinel-1 dem is with respect to MSL.

Is it possible that in Snap all the DEMs are evaluated in Ellipsoid reference and instead the one downloaded is in geoid (vertical datum EGM96)?
This could explain the different height value in the two DEMs.