I think it is because 42 inches is exactly divisible by 8 inches, so you can cut seven 8 inch wide (by 10 inch long) prints across the width with no waste.
I don't understand the 42" either. But do you really need the extra 2"? I mean, I don't think I print over 40". I guess it all depends what you're doing though.
I'm glad I didn't post this.:-)
Wow, thanks for the heads-up on Atlex.com!It seems crazy that a Canon 8300 is only $400 more than the 6300, and now I'm considering a 44" printer just for kicks.