From answer of Lucas Furman:
“By default navmesh walking mode ignores all collisions with world geometry (UCharacterMovementComponent::SetNavWalkingPhysics) and uses navmesh polys as a ground level. While it’s being faster, it’s also less accurate and depends on navmesh generation parameters. After all, navmesh is just approximation of walkable space and doesn’t need to be identical with ground’s geometry.”
Read more here: Purpose of Project Navmesh Walking? - World Creation - Unreal Engine Forums