This comes up periodically, so:
A. The theoretical board with the grain running absolutely parallel to the face of the board - which doesn't exist.
B. The grain is more like real life with the grain running "up hill". Plane in the direction of the arrow and all will be well.
C. But try to plane "downhill" and all will be misery. Don't do it, unless...
D. You find a board like this, which you will, when you
have to plane "the wrong way" for part of its length. This is when you need your plane well tuned, and expect to have to plane is all directions when you reach the smoothing stage.
Don't laugh, but it also often helps to stroke the board; it'll feel smoother in one direction, and that's the way to plane.
Cheers, Alf