Both circumference and girth is measures around something. Why don't both words mean the same thing?
Definitions. n girth A band or girdle; especially, a band passed under the belly of a horse or other animal, and drawn tight and fastened, to secure a saddle or a pack on its back.
n girth The measure round a person's body or round a pillar, tree, etc.
Circumference refers to a curved geometric figure, especially a circle.