What does this mean may I ask?

I’ve just noticed this difference on my skin that made me question what was going on here. I naturally have tannish skin, which I love, and I realized that on both of my arms I had a slighty darker shade to it on one sides of both arms, like if it mirrors the others. At first ugly t it could be a sun burn, but Ik not entirely sure. I mirroring lines up so perfected it makes me wonder even now. Just thought I should ask and definitely interpret it. You can question me too to find out more about me and what I’m describing as well.