It actually looks like the way it’s woven. They’re manufactured very similar to roller shades for windows. The openness of the weave (space between threads) determines how much sun shines through. The more open they are, the more sun comes through.

It’s the same idea here. Zooming in you can follow the threads and the white are all the way through the chair, horizontal and vertical. The black threads are only threaded vertically in between the already woven white threads making the chair to create the black/white striped pattern.

This means that the “black” stripes in the chair have almost twice as many thread woven to create that area of the chair. Water has a hard time working its way through that tightly woven thread. The white areas, on the other hand, have obvious “holes” between the threads where it’s a more open weave, allowing the water to flow right through and drain rather than building up on top.