Here's why:
* Bay base coat: Bay horses have a base coat of reddish-brown or chestnut.
* Black points: The black points on a bay horse (mane, tail, lower legs) are caused by the extension gene, which is responsible for producing black pigment.
* Incomplete expression: The extension gene doesn't always express itself fully. Sometimes, it's only partially active, leading to lighter black points, or even no black points at all.
So, a bay horse with no black on the legs would still have the bay base coat, but the black points would be absent or extremely diluted. This is a less common variation of the bay color, but it's definitely possible.