It depends when the board was flashed, or in the case of older boards, whether it has since been uopdated or not. Back when the PSP and Millennium series had maximum rates of fire of 15.4bps, the minimum achievable rate of fire was indeed 14bps, as no-one saw the need for a boiard to go any lower.
In 2008, when the rate of fire dropped to 13.3bps, Will released a rate of fire patch that allowed the board to adjust down to 10bps. Old boards could be updated, and new boards shipped with the rate of fire patch already installed. As such, the age of the board, and whether it has been updated or not, will dictate whether the board can only go down to 14bps, or whether it can go down to 10bps.
If you have a newer board, or one that has been updated, these are the rate of fire settings: -