Indeed, it has to do with the squares - or more clearly the threads - per inch. 28 count means it has 28 threads per inch (or spaces to make stitches). Stitching over two means going up diagonally two spaces - so you are essentially covering four squares with your stitch instead of one (two horizontally, two vertically). It means stitching over two on 28 count will turn out the same finished size as over one on fourteen count.
The fabric doesn't appear to be available on this site, but you can click on Cross Stitch Fabric in the left menu and look at other options in 28 count. I believe there's some pretty options that would look nice with this piece in the Jobelan category.