FIRE IN FREEFALL - rare physics experiment

I figure it's the increased air resistance at higher velocity slightly slowing the box's downward acceleration. It's still accelerating downward, but as air resistance increases, that acceleration isn't precisely the opposite of the earth's gravity. The further it falls, the closer it would get to terminal velocity, and the bigger the difference would be between its acceleration and gravity. At terminal velocity, the contents of the box would experience 1G, but it's a smooth ramp-up to the point, even at the relatively low speed you were dropping it at. . . I think. I don't know, I'm not a physicist. ;)