Do you perhaps sleep on that side more? Or, when you are sitting on the couch watching tv, do you lean on that side more? While at work, do you sit in your chair leaning on that side? Notice, throughout the day, if you tend to position your body in a way that covers that arm more than the other one.

Have you tried using lotions to soften that arm? A while ago, my arms were dry, as well as the rest of my body but, my left arm was drier and bumpier than any other part of my body and I changed from my shower soap to Dove CreamOil Body Wash. Basically, once you skin gets that bumpy-ness from being dry, until you find the right way to stop it, it stays that way...at least from my experience. I can't promise that this is your problem but, you might give the CreamOil a shot and see if it takes care of your problem.

Take care and keep in touch!