Why in Safari do the values of window.screen.height and window.screen.availHeight differ between normal and private modes?

In Safari the values of window.screen.height and window.screen.availHeight differ between normal and private modes, but not between each other, in these conditions:

• Only one tab is open,

• The window is stretched across the entire display, and

• The developer console is in a separate window

Externally, the browser control panel does not differ in size, and the dock is hidden.

Why do these values differ between normal and private modes, but not between each other?