Your 4K Blu-ray may never have been 4K in the first place

Generally, physical media beats streaming content. The maximum bitrates supported by a disc will outperform those offered by the best streaming services. This is how a lower-resolution disc can appear better than a movie or TV show streamed in 4K. The three main physical media disc standards are DVD, Blu-ray, and 4K UHD Blu-ray — but not every 4K UHD Blu-ray release is created from a 4K source. Many films use a digital intermediate in the finishing process with a 2K resolution, which is then upscaled to create a 4K UHD Blu-ray disc.