think of it this way
your files that you're seeding are split up into a bunch of pieces, like pages from a book.
say your book has 500 pages. if more than one person is downloading from you, you could send the same page twice, thus still uploading 500 tota pages (ie ratio over 1.0), but you wouldn't have upped all the pages in the book.
there's an option called "super seed" which will only send out each piece of the file once until you hit 1.0, which solves this problem.