"You are right though that money needs to be set aside for bad years - in insurance we call it "reserving", and actually it is already accounted for before the 5-15%. It is stashed away in the loss ratio as "incurred but not reported"

Is not correct. CAT reserves are not related to IBNR. IBNR is a) we know the loss has already occurred b) the policyholder has not reported the loss yet. (Or at least in a probabilistic sense, like the hurricane has landed, and we know it will take 10 days for all the claims to be reported, and that 2 days after landfall, say 20% of claims have been reported, and the other 80% of those hurricane claims will be reported over the next 8 days. So at that moment "2 days after landfall" the actuaries will estimate how much IBNR there is.)

What you described is a CAT reserve...it's a seperate reserve taking into account, say, over a 10 year period, the odds and severity of a CAT risk.

If you still are unclear about the distinction, please consult your local actuary or CPCU :-)