It feels like a mistake.
It makes no sense compared to the tables posted by Pomp: https://www.kidney-international.org/article/S0085-2538(15)52697-5/fulltext
99 is a median of the healthy 18-24.
For 60 years olds, like the median in BoM, it should be close to 73, hum guess what, eGFR levels of EXAMINE.
It is the same method of calculation in both cases?