Fascinating. Is this a poll of what players play or listeners listen to? Bit surprised at the HipHop if it's the former.
It is by US album sales, calculated as follows:
Total Album Project Units = Album Sales + (Song Sales/10) + (On-Demand Audio Streams/1500)
ref: Music album consumption in the U.S. by genre 2018 | Statista
In other words, this is what people in the US are paying to listen to.