According to siliconlottery.com 38% of 9900K are overclockable to 5 GHz. Probably they just decided to select good chips from 9900K at factory, those not exactly new chips.
That's mentioned in the article: "The new Core i9-9900KS uses the same silicon currently in the i9-9900K, but selectively binned in order to achieve 5.0 GHz on every core, all of the time."