As far as I can tell, entries are never deleted from the category table. This means that if a category is added to a page and removed, it will be in the category table forever, even if a category description page never existed.
Its probably safe to say that if a category has no pages in it and has no corresponding description page, that it ceases to be a category, since it doesn't exist in any visible place on the wiki.
On the English Wikipedia, more than 37% of the category table are these non-existent categories.
mysql> SELECT COUNT(*) FROM category LEFT JOIN page ON page_namespace=14 AND page_title=cat_title WHERE page_id IS NULL AND cat_pages=0 AND cat_files=0 AND cat_subcats=0; +----------+ | COUNT(*) | +----------+ | 419270 | +----------+
This is out of 1112968 entries in the category table.