Looking at those images on my calibrated system, the matte boxes aren't really visible, I'd be betting the values for the matte box are around 0,16,0 or less and any values under 16 are displayed as black anyway. On the uncalibrated (overly bright) monitor they stand out like dog's balls.
The left half of the image looks about as green to me as the matte-box as well, so it shows how little variance it takes to have it not match perfectly.