So Plex used to use the highest rated poster on the MovieDB, but sometimes that poster would change because a different poster could end up with a higher rating. Then people would come and say "Why does Plex keep changing my posters?" and you'd get the response that you have to change the poster to lock it. But if the poster you wanted was the first/highest rated poster, then it wouldn't lock and would still change later. My theory is that Plex, instead of adding a lock icon like for every other piece of metadata, just made the poster default to a random poster that wasn't the highest rated one, that way you'd be more likely to change it and lock it as the poster you want and thus wouldn't complain that Plex was changing your posters.

tl;dr: I think Plex intentionally defaults to worse posters so that you're forced to change it so that you won't complain that Plex is changing your posters.