WTH, that page 12 cannon design is terrible. The opening on the back makes so w/e they are putting in would fall off right away, and only two wheels with not extra legs means it wobbles the cannon itself up and down. Just make the feeding hole on the top of the back side and give it an adjustable base to change the vertical axis (for aiming).
I think the art looks different because the quality of the raw is better. Or maybe the artist swapped to digital drawing, that could happen as well.