Those were core set cards
Cards that you did not craft and were given access for free for the year.
The core set just rotated and some cards have been removed and new cards added in
You can read the article linked by Vyraneer above which includes this graphic resume of the core set changes :
https://bnetcmsus-a.akamaihd.net/cms/content_entry_media/0OEPTFZ0757W1739224158613.jpg
If you check your collection you will notice that you now have access to Krag’wa the Frog without ever crafting or opening it in a pack (for example)