That would be because it was Elvis lol! They got the clips from a video or something from the late 60's, cleaned it up and put it in. I don't remember all the details but that's the gist of it.
ETA: Here's a link with a bit of explanation. Apparently there was an impersonator, but they didn't use his face.
HaHa! Yeah I saw that. I found it really cool. I heard about technology doing this awhile back. They are going to be able to put deceased celebs back into the movies again, just the same way that they did with the Elvis stunt on American Idol