Perhaps some of the additional clarity the L90 has over the L500? I don't remember much about coilform dimensions and wire gauge, but the blades are definitely thinner. Bill created the L90 first and preferred it over the L500 sonically, but the market was looking for the L500's "open-coil" appearance, which was associated with the then-common practice among musicians of removing the metal covers from other humbuckers to get back the clarity ruined by eddys.