I’m not sure of the details, but I believe that Zazzle shows the images in different resolutions depending on which place you’re viewing them. I know for sure that if you look at the previews by clicking on the preview box in the bottom right-hand corner of the design tool you get lower resolution versions than you get on the product page - someone from Zazzle confirmed that once in some thread that I’ll never find even if I searched high & low. I’m not sure about the resolution within the design tool.
My guess (only a guess) is that they’re trying to maximize the site performance & speed so they’re making some sacrifices in terms of the on-screen display, but I doubt it has an impact on the final product.