I'm a little confused with something and wondered if someone could enlighten me...

There is a very slight difference but in practice that weight rule works. However what it doesn't tell you is that to compare "like for like" you would need more layers of fibreglass and hence more resin to make your fibreglass part as stiff as the carbon fibre part so that is where the weight saving comes in.

Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant