What would cause a photo to appear pixelated on a digital photo frame?

I’m loading photos from my computer onto a digital photo frame. Some of them look fabulous on the frame, while others appear pixelated even though they looked fine on my computer screen. What’s puzzling me is that I can’t figure out anything fundamentally different between the photos that look good on the frame and the ones that don’t. They originated from the same digital camera, have the same dimensions (height and width) and resolution (pixels per inch), and are all jpegs saved at the maximum image quality. Can you think of any variables that I might be overlooking that would cause some, but not all, of my photos to appear pixelated on a digital photo frame?