They just assume all oak flooring is the same. For oak flooring, there are 3 main grades: select, Number 1 common and Number 2 common. Select grade is generally considered the highest grade as it has the least color variation. It costs the more.
This is followed by No 1 which has more color variation and some occasional knots, and that is followed by No 2 which has a lot more color variation and knots. Sometimes, this is called character grade or cabin grade wood.
The differences in the color among the boards are more obvious when the wood is natural (i.e. without a stain). The darker the stain, the less you notice the color difference in the boards.