The same could be said of much private medical information. Guessability, or someone else knowing the information, are not the controlling factors in what is considered private information.

That doesn't mean it's not private. Lots of private information is known by lots of people. Your health record is known by your doctors and the government presumably. Government knows everything about my taxes, my date of birth, national ID (or SSN in US) etc. That doesn't mean it's not a private information. I don't want to share my salary details with other people unless I choose so.

There're plenty of ways to learn of one's SSN or birthday date too - this is still private information, which doesn't concern other people, unless the person explicitly wants to share it.

It's not private from payroll, but everybody knows that, and payroll is expected to exercise discretion.

Precisely the people with hard-to-guess salaries may value that privacy.