45 pounds is the listed weight for the Sport -- not the Sport Hybrid. But I confess I don't think your kayak should weigh more than about 50 pounds. The listed weights tend to assume builders who are very experienced and always weight-conscious. Most of us use more epoxy than is really necessary. I know I did! Epoxy is heavy stuff.

I can't comment specifically on the Shearwater Sport, but I know that my first two S&G builds (Ch16LT & Ch 17LT) came in about 5-6# heavier then spec.  As Birch says, epoxy is the usual culprit.  My fillets were way too thick, my endpours where way to big and I used way too much epoxy when I wet out the glass and on fill coats.  The second culprit is additional gear, like upgrades seats, foot pegs, skegs and rudders.  Weight from extra paint and rub strips also add to that.

My third boat (S&G PP) was only 1.6# over, and that was mostly the seat and rub strips.

I can't really say how you ended up 15# over but I doubt it was the hybrid deck.  Cedar is much lighter than ply so a hybrid boat should be lighter.