Ok. Now I know. When doing a project with pre-cut fabrics, make sure all the pieces are from the same manufacturer. In my quilt along fabrics, the purple strips from Island Batik are about an eighth of an inch wider than the cream strips from Timeless Treasures. Since this would throw off the corner alignments when it comes time to do the piecing, I'm trimming them down as I go. Hopefully this works. And in the future, I'll stick to one manufacturer at a time since this fiddly stuff is the sort of thing buying pre-cut fabric is supposed to avoid. Anyone have a use for fabric confetti?
I'm part way through assembling "strip set b" (not sure why I started with the b's instead of the a's - probably in how I organized the set groups). Pictures coming soon, I promise.