They probably just use BRK.B for most HFT involving BRK. The one-way transfer from Class A to B shares (and the different voting rights) reduces the supply of C

They probably just use BRK.B for most HFT involving BRK. The one-way transfer from Class A to B shares (and the different voting rights) reduces the supply of Class A shares due to a myriad of reasons. This makes the daily volume for Class A shares lower than Class B (when normalized by the trade ceiling imposed by Berkshire).