Don't forget you have your lamps sitting right on the glass. The height of the light above the water will make a difference in the overlap. I don't see any areas in my "18 inches front-to-back" 40b that are lighter or darker. The length of my tank is the same as yours (36"). I bet if you lifted your lamps you'd see a big difference.
I just checked and the bottom of the CFL is 8.5" from the water surface. The bottom edge of the shade is 4" from the water surface. So although these shades are deeper than the shades you're using, the spread is much better at the height they're at.
Also be aware that Fluker make these same lamps, but the inside of the shade is white. I chose the Zoo Meds specifically because they were aluminum inside. Just as a FYI, I'm using 15w 6500K daylights in those. They're generic Lowes house brand. Months ago when I was running DIY CO2 I tried 23w "natural light" daylight bulbs and it was way too much light. I was combating every algae known to man. These bulbs seem to have all the PAR they need to get the job done.
So jart, the take-away of all this is that "it depends"