Actually, his rating didn't drop between September and December. His ranking did because lots of players had their ratings bumped up.
9/21/23: Rating- 96, 4 star, # 4 QB Nationally, # 5 Player GA, #33 overall Nationally.
Today: Rating - 96, 4 star, #4 QB Nationally, #8 Player GA, #71 overall Nationally
If you read the article on the site homepage, 13 new guys got their 5th star, and a lot of other guys in the top 100 also saw their ratings go up, including Nelson, who now is rated at 97