The print graph for SNR is almost identical to the D800 but the 5dIII is rated much lower.
This is easy to understand if you consider that:
"The DxO Mark Sports Score is the maximum ISO for which the three criteria (SNR ≥ 30 dB, DR ≥ 9 EV, CS ≥ 18 bits) are still met.
So all 3 criteria must be met
Since the SNR performance is close, that's not holding the 5DIII back.
Neither is the DR at this ISO level where both sensors are close. It's the weaker CS (Colour Sensitivity) that causes the Canon's Sports (ISO) Score to be lower.
The ISO has to be lowered in this instance until the CS = 18 bits to pass all 3 criteria tests.
Why is the Canon inferior to the Nikon in this regard? Although DXO has mentioned this in the past (http://www.dxomark.com/index.php/Publications/DxOMark-Insights/Canon-500D-T1i-vs.-Nikon-D5000/Color-blindness-sensor-quality), it's worthwhile bringing it up again: Canon's Red channel filter is not very selective, requiring more signal processing and therefore noisier reds:
Compare the R[sub]raw[/sub] colour matrix lines (R[sub]sRGB[/sub], G[sub]sRGB[/sub], B[sub]sRGB[/sub]):
1.94 -0.86 -0.08
2.27 -1.42 0.15
As it understand it, the smaller the positive & negative co-efficient values are, the better. A less discriminating colour filter is purported to offer a bit better light sensitivity (more photons reaching the sensel), but that's not reflected in their respective QE differences here:
5DIII 49% according to www.sensorgen.info
The superior QE purformance is due I believe to Nikon's lead in micro-lens & coating technology.
Note: I believe, SNR and thus QE, is measured on the Green channel (the colour our eye is most sensitive to), so QE measurements alone may not notice any improvement in Red channel sensitivity.
The D800's Sport Score seems to be coming from its SNR figure. Consider the SNR18% at a measured ISO2211: 31.3dB.
SNR ideally is inversely proportional to log(ISO). To compare the "Sports Score" of ISO2853 to ISO2211
= 10 log(2211/2853)
So the SNR at the higher ISO should be 31.3dB - 1.1dB = 30.2dB (approx 30dB).