Repigmentation Occurring Six Months After a Dysplastic Nevus (With Severe Changes) Removed?

Today, at my follow up, my dermatologist re-excised the pigmentation that returned in the center of the scar left from the first excision. My first biopsy results from the original nevus showed moderate to severe changes, but all the borders were clear. What could cause this recurrence of pigmentation on the scar tissue? Could a pathologist mistake "a dysplastic nevus with severe cell changes" if the original nevus was in fact malignant? I don't know how different the two would look on a slide.