Numbers not matching up for the average columns across all rows. There's an assumption I've missed somewhere. I'm not sure if it's how I've constructed my seasons. They seem to check out to me.. 🕵️.
Thankfully, I seem to have the same data as the solution file. I just had to look at what the dates of the season changes were for that year (2012), and applied the correct end date to the 'fall' season.