There was artificial light in the hallway and furnace rooms, one, or perhaps two bulbs; the typical ceiling height for a house is 8' but basement ceilings sometimes are lower. If the wine cellar door is 6'-8" high, as is typical for doors, then judging from the photographs, the basement ceiling is at 7'-6". Fleet was about 6'-4" tall (?) and a pretty big fellow, at least according to John Ramsey. The child's body was on the floor in the wine cellar, mostly hidden by the L turn in the wine cellar. That means that most of the light that might have been blocked from illuminating the wine cellar floor at least. I think a case could also be made for his eyes not adjusting to seeing into a dark room, called dark adaptation, similar to walking into a darkened theatre where for a moment or two you don't see anything unless the screen is lit up and illuminating the chairs and patrons.

Conversely, at 1:00 pm. there was ambient light entering the room. John is much shorter, I think about 5'-9". When he opened the door there would have been some light in the room reflected into the wine cellar. I honestly feel this explains that much of what happened in the basement.

http://b.heart.50megs.com/ramsey/Light_and_the_WR.htm