Does the left side get more frequent sun exposure, like when you stop and lean the bike again?
BITD I frequently did the same out and back ride on an MUP. Because of timing, I usually started the return when the sun was higher in the sky. My right arm would be slightly darker than my left. Sun really takes its toll. I have a jersey that I have worn during several one and two week tours. The upper back area is noticeably faded from UV rays. My rear pannier tops are also quite faded.