Ah, I see the problem. Perhaps it should be that stars each have a specified threshold, for example:
5 stars = less than or equal to 3 hours
4 stars = less than or equal to 3h10m
1 star = over 3h30m
What do you think? This would have to be made clear somewhere in an always-viewed section of the jam so people actually follow it!
But in any case I have changed my mine such that I think the new rating system for it should stay, because it is more important to learn as a developer than to receive an accurate reward for only 3 hours of effort.