The digital signal is not fed straight to the viewing screen. It has to be decoded and processed before being sent to the sound and video circuits. Different decoders use different algorithms and work at different speeds so that explains some of the difference. A certain amount of error correction is built in so that could also add further delay depending on the quality of the signal received by each TV. It is also possible for the sound to get out of sync with the video. When that happens we just switch to another channel and back again.