Newgrounds is a site where people can critique and rate your work. It's not like other sites where you post your art and wait for likes. Users will rate your work based on its content or style, whether it's art, music, games, or videos. They can even save it to their favorites or playlists.
The site can also hide your votes and favorites when you post something like a sketch or incomplete art, making it difficult to see who rated it and how many likes it got.
Additionally, this site can't be hidden; your favorites will always be public and visible on your account. Even when you comment, your reviews will appear, indicating how you rated the user's post.
It may be difficult, but that's how the site works.
Yatsufusa
It's hard to speculate about the reasons without being able to see how the star rating came together.
On top of that, everyone rates art different. Some only rate "0 or 5", others take a more nuanced approach. It's a whole thing:
https://www.newgrounds.com/bbs/topic/1540948
That being said, and I mean no offense, I would not describe your artistic output on the Art Portal as being "above average". Compare your own drawings to those you put in your own favorites to see where you could make improvements: https://fridaychoco.newgrounds.com/favorites/art