Good eye!
I had to go back and check. Turns out the difference was achieved because the game uses two objects (as usual) to create the exit, the top part with the candles and the bottom part that contains the candleholders--and (here's the trick) the two parts actually overlap slightly at where the candleholders are (the yellow). The fake exits have bottom part overlaid over on top of the top part, covering up small portions of the candles so you see more of the yellow candleholders, while the real one has the top part overlaid over of the bottom part instead, so you see more of the candles.
That being said, I actually wonder whether this was intentional or not. The overlap of the exit parts seem to happen in all other Xmas levels, so it's not special treatment specifically for that level. And as we know, the game engine achieves fake objects based on essentially the order they are specified in the level data (first 16 objects are real, after that they're fake), and it's the same order for which the objects are drawn onto the level area. This means the fake objects have to come after the real ones in order to be fake, and thus have to be drawn after (and therefore overlaid on top of) the real stuff, which would result in the visual difference.
Now, they most certainly could've reorder the top and bottom parts of the real exit in the level data so that the real exit would look the same as the fakes current do; the question we won't know the answer to is: did they not realize this subtle side effect of how they made fake objects work, or did they realize it and intentionally leave the difference in?
