Audio and video are encoded, compressed, and transmitted differently because of how humans audially/visually decode them. We fare better dropping late video frames than degrading their quality, where the opposite is true for audio. As we transmit the two as separate, asynchronous signals, it's unsurprising that they are frequently out of sync.
This, but also at the physical devices level, there will be different delays. For example my video stream goes to the projector with some specific frame delay. But the audio goes over bluetooth to a speaker, which introduces its own per-packet delay. The source tries to balance those as much as possible, but the result will differ slightly each time they're turned on.
they follow different channels and at some point the sync is lost...youll get this, for instance, when a synced stream is diverted for additional commentators audio...the commentator and you see the original video, the commentator hears the original audio, you hear the delayed original audio mixed with the commentators audio
