They don't.
A quote from your own link: "All other rating factors being the same, two people of different ethnicities who live in the same postcode will pay the same premium for their car insurance".
However higher premiums are being charged to customers who live in those areas where the local populations are predominantly black or Asian than to customers who live in areas where the local populations are predominantly white.
So, for example, in London people who live in areas such as Brixton or Southall are likely to be paying significantly more than those who live in Islington or Richmond-upon-Thames.