Originally Posted by Despos
This happens more in RTW because of how they sew the sleeves onto the jacket.
Actually, it is more sinister than that (although the rest of his explanation is spot-on). It is a result of what we call "hanger appeal"; the desire to have a garment look nice on a hanger, in a store. But first, a picture pulled from another thread on the same problem.
If it were a pitch problem there would be breaking near the red line in the photo but it looks ok. The sleeve is tight front to back as shown by the arrow. To correct this the sleeve should be cut a little wider and the cap a little shorter (the red line is the original, the black is the corrected version).
Here's the sinister part; there is only so much fullness that can be sewn into the armhole. A shorter, wider sleeve will fit better, but a sleeve with a short cap will tend to lift away from the garment on a hanger. A very wide sleeve cap must be set absolutely perfectly in order to avoid any kind of puckering when on a hanger (the puckering disappears when worn). Most manufacturers err on the long and narrow side so that their garments look better on a hanger, especially those horrible little "wishbone" things that many retailers prefer. I prefer to cut a wide sleeve and use a wider hanger instead, but many stores object because it takes up precious space on the floor. I am constantly having this argument with retailers.......