Why the smaller needle?
Why do patterns often call for a smaller needle on necklines, cuffs, and edgings?
Using a smaller needle when doing garter stitch and ribbed edgings on hats, sleeves, sweater bottoms etc. helps keep your tension correct. Garter stitch and ribbing both are looser by design than stockinette therefore changing your needle size helps to keep the gauge even and to pull in as ribbing should. Typically you use 1-2 sizes smaller needle than what you work the rest of the project in.