So why should the expected OG change when I modify such loss factors by changing the "Batch Size" on Beersmith then?
I think the expected OG is directly tied to Batch Size and the EE%, with any losses already factored in.
I leave 1.2 gallons in the boiler and enter that as loss. Batch size is 4.25 gallons. That tells BSmith I need to finish with 5.45 gallons post boil. My evap rate tells BSmith what my collected wort pre-boil target must be to have 5.45 gallons left post-boil.
That 1.2 gallons I don't ferment still gets sparged and still gets concentrated by the boil. My lautering efficiency is usually 79%, but my overall efficiency is bad, like 57%, because I 'choose' to lose 20% by leaving it in the boiler. If I did not "modify such loss factors" in BeerSmith, my calculations would be way off. With a "lossless system" my brewhouse efficiency would be around 79%.
Let's assume a loss-less system and batch size of 5 gallons. Plan to collect six, boil away one, finish with five. Spill one on floor. If you don't enter the loss as lost, your EE% looks horrible. As a loss, it is accounted for, and your performance looks better.
Not unlike mega corp taking a one-time accounting charge for some failed merger or other nonsense. If it is accounted for, regular operations look better.