Why an 18-gun sloop could carry more than 18 guns

How could a rated 18-gun sloop carry more than 18 guns?

The rating counted the gun ports pierced into the hull, while additional carronades or lighter weapons could fire over the upper-deck rails without changing that rating.

A ship's nominal rating described how many guns her hull was pierced to carry, not necessarily every weapon mounted aboard. Heavy long guns generally occupied those ports, while carronades and other weapons could be added on the quarterdeck or forecastle and fired over the rails. That convention explains why Cherub's listed battery could exceed the number implied by an 18-gun classification. The watch point is useful when comparing paper ratings with the actual broadside available in battle.