Always assumed the increase was for 2 reasons:The boiling bursts small hard starch granules and makes that available to the enzymes, so higher conversion efficiency. Kai has information on his page, and a microscope picture of a granule.
1. You usually use a higher water/grist ratio
2. You increase mixing during the mash
I also assumed I was seeing the increase because I had poor efficiency in the first place (~63%). Potentially a system with a higher average efficiency wouldn't see a difference...
Some starch does not gelatinize at typical mash temps, but I believe that this boost in efficiency is small for barley. I believe the bigger effect comes from the additional mixing when the single infusion conversion efficiency is low, typically due to coarse grind.