The graph nicely shows that XP-L has less current droop (from better process/design) but also a worse I-V diode characteristic (from smaller junction area). From eyeballing the curves the two effects roughly cancel out, giving XP-L and XM-L2 about equal efficiency. Speculating a little, it seems Cree is using it's superior process to cut costs by using smaller dies, instead of making higher efficiency products. Intel has a similar strategy of 'holding back' when there is no competition in the high performance segment.