Updating Atahualpa without using a childtheme

I just updated the Atahualpa theme to version 3.7.10 without using a childtheme. To my surprise nothing really changed in the layout of my website, allthough I made some major stylesheet-changes in the former theme version. Almost every change I made before updating was taken into the updated theme. I just had to re-add the logo image in the header and add read more links to the excerpts of the posts. Is it normal that a changed lay-out is taken automatically into the update without using childthemes, and if so: why using a childtheme when updating?