I would think that if the safe is on a floor or two above the basement and the bottom of the safe is seeing flames that it's a fair bet that the safe will wind up in the basement at some point. Curious if it's rated for a 1-2 story drop when loaded?Are you asking why the factory would leave the air-gap?
Or why I would suggest adding more insulation?
I'm going to assume the first. It's purely cost-based. When they test these safes they set the flat on a floor and blast a fire at the front, back, sides, and top. The bottom doesn't get blasted by the fire. Their only interest is getting the safe to pass the test and therefore they don't worry too much about insulating the bottom more than one sheet. For an application where this is sitting flat on a solid concrete floor (as in their test) it's not a problem. However, if you put this in a 2nd story, there is a chance that a fire in the basement will be able to heat the bottom of the safe. That's a problem. The air-gap may help a bit with moisture but I'm not seeing how it really could make much difference.
I'd definitely be interested in getting one of these if I could afford one at the present time.