In non UL rated safes, the floors are always lightly insulated, if at all. The bottom 2" of the safe will not get very hot in a fire. As far as why they use a raised floor, and not the cement fill, it's due to the construction method. The body of the safe, minus the bottom, is set upside down. The material is pumped in from the bottom, then the bottom of the safe is added. This is normal, and is also seen in most other gun safes out there. When you're building something out of sheet steel, it's not going to be completely square. Difference between plate and sheet.