You are looking at the additional images, which are controlled by completely different selectors.
#productAdditionalImages is the whole block of images;
.additionalImage covers each individual image.
In my 800x600 monitor, they overlap by 50px or so and the right one spills over the sidebox. Maybe your images are too big to fit in the probably 33% width the code is specifying? Have you tried two images per row?



