I think it’s because usually when we are working something in stocking stitch, we are used to having the edge stitch in garter. In this case, keep it in stocking stitch. Knit on the knit side and purl on the purl side. And yes, it will curl. But later on, you’re going to pick up stitches and add an edging that will flatten out the curl.
So since you’re doing Cityscape, you’ll cast on 52 stitches in Ultramarine. The repeat is 50 stitches so you have one extra stitch at each end of the row. Knit the first stitch in Ultramarine (for the edge) and then knit 10 stitches from the chart. Change to Aster and knit 20 stitches from the chart. Change to Ciel and knit 20 stitches from the chart. Then knit the last stitch in Ciel. Turn it over, purl the edge stitch in Ciel then purl the rest of the chart until you come to the edge stitch. You’ll be using Ultramarine. Just purl that in Ultramarine.
It does seem a little weird for it stay stocking stitch since it’s only one stitch but really it’s just knit one side, purl the other side no matter how many stitches you have.
Hope this helps.