The graph confused me for a moment, but then realized the graph is showing the winter with SNOW on the ground, not the general case. So in the winter the vertical panels produce more power than tilted panels in very specific conditions and depending on how often that occurs, it may make up for the loss in efficiency during other times of the year. I'm guessing the vertical panels gain some advantage of the sun reflecting off the snow and into the vertical panel that the tilted panels do not, especially when the sun is at low angles to the horizon.
