Tallying categories in numbers and sorting

I was recently pulling my hair out when I realized that all my formulas in my summary table were incorrect after re-sorting my source table. I couldn't find the solution in these forums, but finally found a video on YouTube that properly explains it and wanted to share because I'm sure other people must be having this problem.

https://www.youtube.com/watch?v=KU-nYXO7fPg