Single product on category page sitting alone on a row, while there are four products on the rows ab

Hi there

I am experiencing an odd problem on a site that's currently under development.

On (at least) one of the product category pages, a single product is alone on a row while each of the rows above and below are "full" (four products per row). Same product type, same product category.

Here's a link to the page:

Any ideas?