DEM SRTM data High Altitude different to Oruxmap

Hi,
I am just wondering about two differnt numbers for the altitude in the same Place.
Oruxmap shows using Altitude from DEM data i.e. 97 meters and the app "High Altitude" shows 94 meters,
at the same Long/Lat Position, and using the same .hgt file in the same folder.
When you move the pointer on a contour line in the map, Oruxmap shows always a differens of about 4-5 meters,
but "High Altitude" shows the correct altitude on the contour line, again with the same .hgt file.
????

Regards