The two cores only share FP and vector execution units. A ton of code doesn't use those at all, and so they are effectively two separate cores most of the time.

The two cores only share FP and vector execution units. A ton of code doesn't use those at all, and so they are effectively two separate cores most of the time. It provides full compatibility on both cores, full performance very often, and saves a lot of die area (FP and vector are going to be very transistor-heavy). It's just a tradeoff.