The mono version has a lot more weight at the bottom because overall density there is much higher, the shadow areas on the porch have fallen into very deep shadows. The green coloration itself fills in the shadows and therefore lightens up that area. Perhaps it all comes down to visual weight. Would be interesting to see how different approaches to monochrome conversion would influence the effect, such as if green was converted to a much higher gray value.
If I cover one of the images and look at them one at a time, while trying to remember the other, I don't get the same impression.
Makes me think I need to go back and read up on color theory. Another similarly striking thing is how having complimentary colors in the scene can add snap to both colors, even when the area covered by one is very small. If you selectively desaturate just one of the colors, they both lose impact. But wait, there's more...