I had a problem with this pattern as well. I have been knitting the fourth size so I have been following the size that was printed in orange. When I went to sew it up ready to do the neckband one side of the back cast off was wider than the other, so I read the details again very carefully.
I had 121 stitches on the needle, which was correct from where the increases took place after the ribbing.
Yet the instructions given for shaping the shoulders told me to:
Cast off 39 sts at the beginning of the next two rows (which makes 78 in total) and slip the remaining 61 sts onto a spare needle - that makes a total of 139 to me, which just happens to coincide with the number of rows that had been knitted.
Has anyone else noticed this or was it just this size that there was an error.