I’m not sure about your visual interpretation, but I completely agree that the two scales don’t translate directly, and that if something is rated 7/10 I’d assume it’s better than something rated 3.5 stars / 5.

As to the reason? I wonder if the scales five different senses of the middle value? In a five star system, 3/5 film is the middle value, and not especially good nor bad, but I’d probably give the same “totally average, not good not bad” film 5/10. Similarly, it seems weird to translate “Awful, 1/5” into “Awful, 2/10”. So maybe the difference comes from a lack of clarity about half stars, it’s okay to give 0.5 / 5? But not 0? Or 5.5?

And that doesn’t even start to address the modern “if it’s rated less than 4.6* it’s probably awful” issue…