Before it wasn't measured, if the winner made the game for over 3 hours, the game would be disqualified from winning. However, there have been cases when games were made in 3 hours and 5 minutes, 10 minutes, which on one hand is insignificant, but there is disagreement, should it be counted or not. This is why we added the rating, so that people could make their decision, whether it's bad or good enough. Aggregated rating should be more precise, however when a game has been made in 3 hours and is rated less is weird. :D No idea why people would rate it less. That said, other ratings are also not set properly often times, for example, when art assets or sound assets are used from other sources. People should consider it and put less than 5 stars, which doesn't happen always. What do you do to make it objective? The best option in my opinion is to focus on your growth as a game developer and do your best to rate the games properly.
If you have some good ideas to improve the current rating system, you're welcomed to share.