Are the gathers there to allow the seam allowances to match but when you sew you almost sew in an eased sleeve. I haven’t made the jumprope, but I noticed when I was making my harry potter robes from the narture walk pullover pattern that I needed to gather the sleeve cap just slightly to make the sleeve fit, I only discovered this after making the first two and wondering why I had to make so many clips into the seam allowance, THEN I read the instructions.
Just easing the sleeves in and cliping the seam allowance http://www.flickr.com/photos/73669617@N07/8226632742/in/set-72157631749713525
Slightly gathering the sleeve to help it fit better into the arm hole oppenig and all the gathers ended up in the seam allowance.
http://www.flickr.com/photos/73669617@N07/8335360131/in/set-72157631749713525
You can see there are no gathers on the second one on the outside, all the gathers ended up in the seam allowance.
Hope this helps.