What causes skin discoloration after facial hair/beard transplant? (Photo)

I received a facial hair transplant one year ago. Appx 2000 grafts. Since that time the receipt the area has been a darker hue, noticeably outlining the beard. What has caused this? My theory is either doctor went too deep with grafts causing bruising, hemosiderin staining or skin wasn't cleaned off grafts or sun damage. Not sure but I've seen other cases and would be curious to hear doctors opinions. I'm considering another one and don't want the same damage.