Why when asked to create a "background size to 20 pixels square" do you need 2 sets of "20px" & not just one?

I thought that in shortcode, if you wanted all the sizes to be the same on all four sides of an element, you set the first value, and that would count for top, right, bottom, and left sizes. In the quiz, though, you have to specify both the top and bottom to 20px and the left and right as 20px. So instead of just the shorthand " background-size: 20px; " you have to say " background-size: 20px 20px; "

They're the same, so why two of them? I could see if you needed two different sizes (20px for top and bottom and 40px for left and right) or needed just the left and right to be 20px (" 0 20px; ") but I thought the first number would carry across all values if no others were specified, thus making the element square.