Behavior I don't understand in bash

I have a folder with 3 dummy files: ab0, ab1 and ab2.

$ echo ab*
ab0 ab1 ab2

$ myvariable=ab*
$ echo $myvariable
ab0 ab1 ab2

$ echo 'ab*'
ab*

Up to here, I think I understand. But:

$ myvariable='ab*'
$ echo $myvariable
ab0 ab1 ab2

I was expecting ab*. This means that there is a basic that I don't understand.

I've been searching for single vs double quotes, expansion and more in bash tutorials and manuals but I don't get it yet.