What happens if you tilt your head horizontally?

Try it!

Now the raspberry works sideways - the monitor seems to jump left to right. But it still scans from top to bottom, using horizontal lines.

Suppose the raspberry shifts the screen:

to the right when it's painting the top part to the left when it's painting the middle to the right again when it's painting the bottom

That will cause a wavy distortion, like the one above.