there is two diferent measurements for duration on a cam, there is totalduration wich i gues your engine builder is refering to and then there is duration at 0.050lift wich rumblefish is refering to
a quick search at summits website showed that 230 at 0.050 lift matches the 280duration your engine builder refered to ;) this whas on compcams extrem energy series cams..
hope it helps