From the link: "Young drivers tend to need less coverage than motorists with families or greater savings to protect, a factor that we included in our analysis."

From the link: "Young drivers tend to need less coverage than motorists with families or greater savings to protect, a factor that we included in our analysis." They are literally correcting for the fact that young driver's drive cheaper cars less distance. It's definitely true in the US that car insurance is more expensive for younger drivers when all other aspects are held fixed.