The code isn't saying that wire will be installed in an ambient temperature of 75 degrees C, its saying that wire can be allowed to reach 75 degrees C. And table 310.16 gives you the amperages that will achieve that temperature assuming an ambiant temperature of 30 degrees C.
If the ambiant is higher, ofcourse it will take less amperage to reach the threshold temperature of the wire so thats what the correction factors at the bottom of the table are about.
As far as GFI's outside, the code does not require a GFI installed outside, but a GFCI protected outlet. And that is purely for shock protection due to water.
