Question about equipment profiles/boil off

First post here  I am a fairly new all-grain brewer with around 4 batches brewed so far.

My all-grain setup right now consists of a 48qt Coleman cooler mash tun and a 8 gallon kettle.  One of the equipment profiles in BeerSmith is a 7.5 gallon kettle and 48 qt cooler.  Using this it shows a boil volume 6.41 gallons.  I have had good results with this and always end up with right around 5 gallons of wort.  I recently purchased a 5 gallon Igloo cooler with a false bottom which is another one of the equipment profiles in BeerSmith.  Switching my recipes over to this new equipment its showing 6.0 gallons boil volume, maybe I am missing something but why would my pre-boil volume now be .41 gallons less just by using a new mash tun?  My boil off rate shouldn't be any different.

This is only speculation, but I think they expect your loss on lautering to be less in a cylindrical vessel with a false bottom than in a rectangular vessel with some kind of manifold collection system.  Therefore, you need less water to start.