Number of horizontal images changes when re-loading a page

When I'm on a page of search results and refresh, or when I click in to an image and then go back out into the search results, the number of images that are displayed horizontally changes, like the site thinks that the horizontal width of my browser (or my desktop resolution) has changed. This is inconsistent behaviour.

Has anyone else experienced this? I'm running Dissenter, which is based on Chrome.