Brewhouse efficiency is the percentage of a grain bill's theoretical maximum extract that actually makes it into the fermenter — a single number that bundles together every loss along the way: incomplete starch conversion in the mash, sugar left behind in the spent grain bed, and any dead space or trapped wort that never reaches the boil kettle.
Why the same recipe software gives different results for different brewers
Because efficiency is equipment- and technique-specific, two brewers using an identical grain bill and identical recipe software can hit meaningfully different original gravities, simply because one setup runs at 65% efficiency and another at 78%. Recipe software needs a brewer's own historical efficiency number to predict OG accurately — a generic default is a starting guess, not a substitute for tracking your own actual numbers over a few batches.
Distinct from mash efficiency, on purpose
Brewhouse efficiency is the whole-system number; mash efficiency measures only the conversion step in isolation, before any lautering losses are counted. A batch can have excellent mash efficiency and still show disappointing brewhouse efficiency if lautering technique or equipment dead space is losing extract downstream of a perfectly good mash. Tracking both numbers separately over several batches, rather than just the final OG, is what actually reveals where a recipe's shortfall is coming from.