Why is first row of products not aligned?

I have configured my MarketPress eCommerce storefront so that product thumbnails show on product list pages. I have sized them (and titled them) so that I have 4 columns of product images (four per row).

For some reason, the 4th product in the 1st row is not aligned with the others -- it is offset roughly 50px "below" the rest of the line of products.

This only happens in the first row, but not on the 2nd, 3rd, 4th, etc...

I have double-checked everything and cannot determine the cause of this mis-alignment.

I can attach a screenshot if necessary.