Hey @cclawrence, welcome to the Spotify Community!
I've looked into this issue for you, and I think I've found the answer. It doesn't make a whole lot of logical sense, though, so bear with me. Basically, what I've found is that an album counts as 1 track + the number of tracks in the album. So if you add a 10 track album to Your Music, it actually takes up 11 spots. So, if you have around 800 albums, this would (unfortunately) make sense.
If you'd like to see the 10,000 song limit increase, please go add your comments/votes to this idea: https://community.spotify.com/t5/Live-Ideas/Increase-maximum-songs-allowed-in-quot-Your-Music-quot/i...
Please let me know if I can help you with anything else 🙂