Hi Andrew,
Good catch. It looks like the power dissipation on the high side FET was miscalculated at those operating conditions and thus the efficiency was erroneous.
I tried changing the FET in the BOM list to an alternate FET and this seemed to correct the error. You can also try using the different optimization settings to get a BOM you would like. Can you post a BOM you intend to use? I can review it if you would like.