What does the phrase "it's anybody's guess" mean?

I often came across this phrase in books and movies and thought I understood it, until I thought to cross check my understanding.

What happened after that is anybody's guess.

When I read a sentence like the above I thought it meant that anyone can guess what happened after that.

But today when I researched the meaning of the phrase, to my surprise, I found out that it actually means quite the opposite. "Anybody's guess" means "no one knows"!

So when you read or hear "something is anyone's guess" it means no one knows what it is.

Examples:

• When it will rain is anyone's guess. (Right: No one knows when it will rain. Wrong: Anyone can guess when it will rain.)
• It is anybody's guess what's inside the box. (Right: No one knows what's inside the box. Wrong: Anyone can guess what's inside the box.)