Google Accessible Web Search uses relevance, but it also gives priority to pages that function well without images or will otherwise be more useful to the visually impaired. Some Web sites do equally well in both Google Accessible Web Search and in the standard Google search engine.
Behind the Scenes:
Priority goes to sites that "degrade" well, which means they are not visually distracting, they can be navigated by keyboard, and they would tend to work well even without images.
People with small monitors, older operating systems, or handheld computers may find Google Accessible Web Search results more appropriate for their devices.