There is always CPUs that are not functioning correctly after manufacture. So for the quad core line, they have opted to sell those quad core processors that are missing a functional 4th core after manufacture. So, instead of scrapping it, they made another product line.
Intel followed the same principle when trying to get higher clock speeds. Test every CPU for maximum stable clock speed, and then label them accordingly. This happened a lot during the DX4 100, DX2-66 range of CPUs, and early Pentiums.