Why does Matt hardly recognize Attean when he comes back to the cabin in "The Sign of the Beaver"?

There are two instances in The Sign of the Beaver when Matt is startled by Attean's very different appearance. One was a happy moment and the change was mostly physical. The other was not a particularly happy moment for Matt and the change had more to do with Attean's demeanor.

In the first instance, Attean returned to Matt's cabin after the two had killed a bear. Matt sat in the doorway of the cabin and was startled by what he thought was a stranger emerging from the woods. The stranger ended up being Attean with his face brightly painted in preparation for a feast he invited Matt to attend.

The other instance occurred later in the story when Attean and Saknis came to visit Matt at the cabin. Matt knew that Attean had been in the wilderness for the Indian ritual of finding his manitou and becoming a man. When he arrived with Saknis, Matt saw a completely changed, serious Attean whose hair, clothing and demeanor were now very different. His appearance and stance were identical to those of Saknis. Attean had become a man.