I have been working on an offline version of my Django web app and have frequently deleted model instances for a certain ModelX.
I have done this from the admin page
Did you actually drop them from your database or did you delete them using Django? Django won't change AUTO_INCREMENT for your table just by deleting rows from it, so if you want to reset your primary keys, you might have to go into your db and:
ALTER TABLE AUTO_INCREMENT = 1;
(This assumes you're using MySQL or similar).