The deleted categories are still present in the Categories-Combobox of details in the backend.
Examples:
1 .- Delete a category K2-categories
2 .- Edit K2-Item and view categories in Details Backend (right column)

The category "Catálogo Casas vip" is deleted in K2-category, however in the Details is not deleted.
View images for an idea.

Link : casasycosas.eu/

Take a look in Trash Categories. Ther you can empty the trahscan - I had the same thing before.
It is like the Windows Trash, if you delete and wanna recreate you have the option.

Thank you very much Troponin.
I've had the same mistake again. I had completely forgotten.
You've seen the other post?

k2community.joomlaworks.gr/forum/topics/error-in-counting-categories