It's a really good question - I made the note elsewhere [1] that I was really surprised at 663 million users, because that would make Skype more popular than Facebook (by measurements of popularity that don't include swearing the calls that start glitching ten minutes in).
170 million I can believe more readily. It could just be the difference between users and registered accounts (I know several people who've set up Skype 4-5 times over the years, using it rarely and then creating a new account on a new machine when they need to use it again).