Strange all right as its hardly a combi at 15kw!!, It depends on where the cylinder stat is located, its normally say 6" or so above the coil outlet to allow ~ 20L of HW use before the stat cuts in again, do you have a secondary circulating pump?.
Any water flowing into the tundish from the PRVs?.
These questions are relevant, if there is a constant heat loss from the cylinder for some reason, the Boiler will fire up more often. Would help us help you if you can answer them please.