Face skin tone darker than neck skin tone?

Patient: I didn’t know what category this actually would fit under, but I was wondering why my face always becomes darker than my neck a few hours after cleansing? I do have some acne, and I’ve tried my fair share of products. I don’t consider myself to be sensitive skinned, just oily. I would think it’s a pretty significant tone difference, but it’s bothersome to me. I’m wearing sunblock when I leave the house as well.