A degree on a scale, and the flatness of a plane. Both name a position measured against a fixed reference.
Latin libella (a small balance, a level), a diminutive of libra (scale, balance), via Old French livel; a degree on a scale and the flatness of a plane are the same word because both are measured against a fixed reference.
A position measured against a fixed reference, whether the reference is a scale of degree or the plain flatness of the ground.