I think efficiency is based on a calculation that can be manipulated.
For example: according to BeerSmith I get ~70% brewhaus efficiency — No Sparge, Mash in a bag in my MLT, and lauter into a boil kettle when the mash is complete. I leave behind nasty crap in the MLT and the BK.
If I would sparge, or simply dump all the contents of the MLT into the BK, or BK into the fermenter post boil, my efficiency would jump dramatically. …but I don’t want to do that. I want to xfer clear sweet first run wort to the BK, whirlpool and leave the trüb (with an umlaut) behind in the BK in an effort to get clear-ish bitter wort in the fermenter.
I compensate for those losses by simply adding a bit more grain and water. IOW, efficiency is just a number that I use as a benchmark. As long as I hit my intended volume, at my intended SG, in my fermenter that’s all I really care about. Everything I do before that to ensure that happens has been determined thru experience. I ‘over-brew’ to get my desired results — efficiency be damned. It doesn’t bother me that I spend ~$3 more to get there.
Now, having said that, I am not haphazard about my processes. I have gone to the trouble of setting my mill with sieves to ensure I get the right balance of crush v lauter for my system, I am anal about mash pH, I mash based on a test I did a cpl yrs ago vs an arbitrary 60 minutes, I boil at the same power setting each brew, I whirlpool to get as much as I can from the BK, etc…