The perfect nap is on the bed, not on me!
Yes, it might be a fluke. Maybe he preferred my left boob to my right - which is true; he often rejects my right probably due to more forceful letdown - but he didn't want to nurse today. Maybe it just so happened that he got real tired as I changed arms. But then I recalled at least a couple more times when he was more settled in my left arm than in my right. This worked again at bedtime.
Whatever the reason, if he can sleep in my left arm, I'll use that arm to carry him. It's way better than holding him upright which hurts my back like mad.