Hi Samlittle
The efficiency is about light output(measured in lumens) per watt.this thread has a lot of info about everything to do with flashlights,but specifically,near the top,it has luminous flux tables for various emitters including XM-L and XM-L2.The XM-L U3 has 220 to 240 lumens at 700mA,the XM-L2 T6 has 318 to 341 lumens at 700mA,same thing.The XM-L2(edited) U2 has 364 to 386 lumens at 700mA.There’s your higher efficiency,more light output from the same current input.Hope this helps.