How a keyword rank, what's the difference between "best x" and "x"

Hey, guys, hope you're doing great.
I'm a newbie to niche site, and nowadays there's a problem confusing me. For example, a site can rank 1st for keyword "best wireless router", and it also ranks 2nd for keyword "wireless router". I found this not only exist in just a few keywords, is this thing just a random or we can work to achieve it?

umm...

They are optimizing for multiple keywords on one page.

https://moz.com/academy/multiple-keywords