At the edge of a lapel or pocket flap (or patch pocket), there is a narrow band of "tape" that reinforces the edge of the innards. Actually, I know this is true of a lapel, but I don't think there is tape inside a pocket flap. However, there is a narrow band of excess cloth -- the seam allowance -- that has been folded back and tucked under the facing (showing) cloth.
Anyway, this seam allowance (and, where appliciable, tape) is inside the pocket flap or lapel. You can't see it. It's covered by the top cloth that is meant to show. Tailors typically reinforce the edge -- the very edge, milimeters from where it ends -- with a pick stitch that holds all the layers together and maintains a crispness to edge.
However, sometimes they add an additional stitch, a centimeter or two from the edge, that runs parallel to the one at the extreme edge. This second stitch highlights the additional width caused by the seam allowances and the tape, making the edge appear to be a bit swelled or thicker than the rest of the lapel. Actually, on all lapels the edge is thicker because of the tape and the seam allowances. But without that second stitch, you will probably not notice unless you feel it with your fingers.