A different grade of carbon fiber most likely. When I was looking for my Ruby Pro they had a 44cm Expert in stock (this was back when they only had the Pro, Expert and Comp models), and I quizzed the shop about whether or not it might be a good or bad idea to go with the frame they had and just change some components (there were only 9 44cm Pros made that first year so getting hold of one was a gamble....). He told me that the Pro was a different carbon and slightly different construction, and that rather than try to change the Expert I should hold out for the Pro, that I'd end up with more for my money in the end, as it was likely to cost me enough to make up much of the difference to do the swapping I would have wanted (and he found me my Pro too!)