The Nether has a different default block shading from the overworld (and that's intended behavior).

The topsides are brighter and the bottoms darker while in the overworld, but both are brighter in the Nether.

It is assumed that entities should follow the same pattern. However, they have brighter bottoms and darker tops instead of having both displayed brighter.

This causes entities and block entities to feel very out of place (especially piston arms, as they're always attached to blocks).