That's because Rebirth appears to be using PSSR, or PlayStation Spectral Super Resolution, which is Sony's new machine learning-based upsampling solution exclusive to PS5 Pro. The new console packs a substantial increase in GPU grunt, but the cost of native 4K rendering would likely overwhelm the enhanced console. Instead, Square-Enix has opted for this upsampling solution, taking the game from an average internal resolution of roughly 1152p to 1224p in my counts to 4K. I got counts as low as 1080p, and as high as 1296p - though if we were able to get our own capture here we might be able to see some greater variability.