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Those numbers are meaningless. Look at the graphs instead.
Yeah it's always better to look at the graphs at DxO.
There is SOME method to the madness of the D800 getting a higher low ISO score in that they count the more color-blind filters (under natural low lighting, under tungsten I think it becomes more similar) against Canon, which isn't an entirely bad thing otherwise makers might start cheating the filters to absurd degrees (although in this case, it probably gives the D800 a bit of a real world artificial boost as to how things would actually look to the eye). Also the D800 has more MP which means tighter 'grain' which tends to look better to the eye even if it's a bit noisier (not that DxO counts this at all) and it also gives more room to apply advanced NR methods, when DxO normalizes they just do the most simple norm possible, but in the real world you might use advanced NR which could bring a higher MP camera a bit better score relative to a lower MP camera actually. So DxO probably undersells the D800 high ISO look a bit by not counting the extra MP as a bit of an extra bonus and probably oversells it by counting the color sensitivity a bit much (especially since plenty of low light stuff would be under artificial lighting where the difference in that spec I think is just about nothing).
I think the high iso rating number overplays the real world effect a bit much though for the high-iso color sensitivity factor and real world I think the extra DR of the 1DX would outweigh that and matter more, so ignoring the 'grain' size factor, real world I'd say ISO3200 and up the 1DX should get a higher overall score, maybe even from ISO1600. Of course the MP difference makes it more complicated and probably tosses out the ISO1600 and makes the 1DX not pull ahead until at least ISO3200, although it probably depends on the shot, if it has a decent amount of DR and many parts of the scene are near black then the DR of the 1DX I think could toss it above the D800 much more easily at high ISO, for a low light scenes with few parts near black it's probably much closer looking between the two. Overall it's probably a bit odd that the D800 gets the higher high iso rating and probably something to disregard and when you go really high ISO I'd bet the 1DX comes out clearly ahead.