High efficiency happens. My last 5 batches since I last re-adjusted my mill:
Not bragging, just stating a fact that it can be higher than 80-something.
In my experience, the crush has the most to do with it.
Secondly, collection of every last drop of wort and not wasting any in the bottom of the mash tun.
Thirdly, in my particular case, I think another big part of it is because I am usually a small batch brewer, so for instance if I am trying to make 2 gallons of beer, I really need to sparge enough to get 3 gallons, because I'll then boil off about 1 whole gallon, which is like 33% boiloff rate by volume. Most people aren't doing that. So since I need to sparge with a greater volume percentage-wise than the average brewer, I am collecting a greater amount of sugars then too, so I can easily achieve efficiencies >90% on a regular basis.
On the other hand........ those last two batches at 83% and 90% were about 6 gallons each.
Other than that................. any gump can get lucky every once in a while.