I posed this same question to a JVC product manager a few weeks ago. He told me that the native aspect ratio of the imager chips is 17:9. A 16:9 source does not utilize the full native resolution of the chips, thus when zoomed to fill the screen, the unutilized resolution spills light past the screen boarder.
From Wikipedia: In television and consumer media, 3840 × 2160 (4K UHD) is the dominant 4K standard, whereas the movie projection industry uses 4096 × 2160 (DCI 4K).