When you convert an unversioned page into a versioned page, two dot pages are created. One for the space's initial version, and one for whichever version you've selected as your working version in the Version Picker.

Scroll Versions preserves the initial content of the unversioned page in the initial version, while any changes made are applied to the working version. This way, you don't lose track of your initial unversioned content, as it is preserved in the earliest version of the page.