Hi,
I found the answer.
The author in this post on the forum talked about EGM96.
This led me to check the effect of the “Apply Earth Gravitational Model” option in the Terrain-Correction parameters (only available when using an External DEM). The elevation offset did come from there.
There’s still an issue regarding why Google Earth doesn’t provide the same elevation values (despite supposedly also using EGM96), but it’s more for my curiosity than anything else.