As the other poster stated the S in the 9900KS is different. In this case the 9900KS actually draws more power with a higher base and boost clock than the 9900K.
This is a good point, however a real test would be to compare the 3800X vs. the 9900KS with JEDEC memory clocks, so 3200MHz for the 3800X and 2666MHz for the 9900KS and while there would be a difference with a ~500MHz bump for the 9900KS and a ~350MHz reduction for the 3800X, I don't think it's going to make up the ~8% performance difference. Zen was always designed for scaling out to many cores, so it's not surprising that even at a single core disadvantage, the Zen-based parts perform better in multithreaded workloads.