This is an 'efficiency' issue. When brewing all-grain, your recipe is based on a certain efficiency. By plugging in your efficiency, you can accurately predict how much grain you will need to mash to achieve your target original gravity.
For example, my efficiency is 70%. If I am using a recipe from a brewer who gets 80% efficiency, I will need to adjust the recipe to include more grain. The brewing software I use is Beersmith and its easy to adjust the efficiency (and determine your efficiency).
Commercial breweries aim for high efficiency to cut costs. For homebrewers, our volumes are small and the cost isn't such an issue. A consistent efficiency from batch to batch is more important than high efficiency.