*deletes technical answer involving aliasing and sampling rates*
That's probably because when you zoom out, it bunches the lines together. So what you're seeing as one thick line is actually two or three lines squished in to one.
Take a screenshot and post it here, if you still have questions.
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Ahh but it doesn't. With all those lines I'm not even sure the background moves when you scroll.
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I'm fairly sure it does. It's just a repeating pattern, so you can't see it.