The files on TPB generally aren't the original scene releases. They get transcoded to lower qualities by other people for the masses who don't care.

The scene releases are usually the highest quality possible released in exclusively closed private trackers - sometimes only paid accounts are accepted as a member (if at all).

Everything else probably 95% you see in the wild are transcoded to lower bitrate and stripped out of all the extra files in the container (sub/multiple audio stream/highbitrate audio) to smalled files - because thats what most people want:

Small Video easily playable on a regular screen.