There are many examples of tenons that are not centered horizontally. Here's another...
http://www.westcoastpaddler.com/community/userpix/512_0119_1.jpg

Yes, in a five minute scan of the web all the tenons I saw on kayaks being built were off center. The limited historical data I have shows kayaks built with thinner, bent ribs that fitted into sockets. I have seen museum kayaks with the thicker tenon-fastened ribs as well, but was unable to get close enough to see details.

Centering tenon requires a little more precision especially for an off-square joint, so it seems likely that Brian's theory - that's it's easier - is correct. It's a bit counter-intuitive, for me at least, to make a tenon off-center. If this practice is unique to kayak-building it may just be a traditional hold-over from Inuit and other aboriginal practice.