I’m pleased you’re enjoying AYoT! Your cloche looks lovely.
Romi created the design, so I can only speculate about the reasons for her specific choices on these details, and I’m guessing that the k1tbl just looked neater to her.
For the needle sizes, I queried it with her when I was editing as I wondered whether the hat body sts should also have been on smaller needles. Romi let me know that she had knitted the sample with the two different sizes and she was pleased with how it fitted around the brim. The smaller needle on alternate rows was just enough to get it to fit how she envisioned it. So we added the line clarifying that it was intended that the rows were worked with different sized needles.
I hope that helps a bit.