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I could word my question differently: why does EV vs ISO characteristic curves differ so much between Canon and Nikon cameras? Canon usually has a plateau at low ISO values.
As you have noticed, the DR curve for Canon reaches a plateau at the lower ISO settings. The reason for this is read noise caused by sub-standard electronic components on the sensor. There is a constant low-level noise generated by poorly designed electronics, and this low-level noise sets a hard limit on how much detail you can pull out of the shadows.
Whatever extra DR you might've gained in shooting ISO 100 instead of ISO 400 is obliterated by this blanket of constant noise. Note that this operational noise produced by electronic components is very low, but constant; so at higher ISO settings their effects are negligible compared to other, more significant sources of noise. Unfortunately, at the lowest ISO settings this operational noise is essentially the largest source of noise.
This is the reason why the 5D Mark II, for example, doesn't have as much DR as you might've expected at low ISO... given its excellent high ISO performance.
But it would be unfair to attribute this problem strictly to Canon cameras. Based on the DXO charts, it looks like (almost) all other brands out there exhibit this characteristic. But Nikon must've made major advancements in its recent batch of SLRs. They developed it for use with the D3x, then apparently passed down the tech to other cameras since (D90, D5000). That's why these newer Nikons all have DR curves whose slopes don't plateau at the lower ISO ranges. For example, the lowly D5000 boasts higher DR than the mighty D700...