That is strange and interesting as it seems counter-intuitive since a B.A. in say, physics, is less rigorous than a B.S. from a pure physics/mathematics standpoint.
Few would argue that a BA in physics (which would actually be in Natural Sciences) from Cambridge is "less rigorous"
¹ There's a fee...
² https://en.m.wikipedia.org/wiki/Master_of_Arts_(Oxford,_Camb...