I checked on my test installation (fresh v1.3.7), and I get the category showing with the same tags you have, but the normal individual category description above the listing. You seem to have the define_main_page text above the listing, instead of the category description, but labeled as category description.
You say it is category 18, but I don't see any category with that id in the categories menu. What is the name of your category 18?
The products in your home page display are in several different categories; it is not a category display you are showing, but the specials or featured centerbox.
Maybe the SEO mod you have active is doing something incorrect to the page display.