The ground shadow has light on it. The underside of the log has no light on it. That’s why they are different.
But why so dark and how to fix it?
Dark because there’s no light under there. Nobody is ever going to see under there.
If you’re really worried about it, you can put a point light there, or increase the skylight.
is the log a static mesh or a skeleton mesh? lights and shadows are complex
You have cast shadow on the ground and probably self shadow on the logs. two different kinds of shadows i think