I think you have a misunderstanding of what the sensor ISO means. There are two fundamentally different ways of defining ISO.
1) The industry-standard definition of sensor ISO is based on the amount of light it takes to saturate the sensor (produce a value equal to pure white). That's important to know, because any exposures greater than that just get clipped. It can produce counterintuitive values. If you double the efficiency with which the sensor captures photons and converts them to photoelectrons, leaving all its other characteristics unchanged, then it only takes half as much light to saturate the sensor and you've doubled the ISO. That make sense.
But… Suppose you make a sensor that just has larger pixels or deeper wells, so that it can hold more photoelectrons. Not changing anything else about it, just giving it a longer dynamic range. What happens then? Well, it takes more light to saturate the sensor. Which means its sensor ISO **DROPS**! But it still has the same sensitivity to light at any given level. WTF?
As I said, counterintuitive.
This is one reason why many large format digital backs, with their huge pixels and huge dynamic range (which means a huge exposure range) come in with such low ISO values. Their pixels aren't insensitive, but they are designed to be huge light buckets, so it takes a lot of light to fill them.
2) The industry-standard definition for camera ISO, and the one that makes the most intuitive sense to photographers, is based on evaluating real photographs to determine which exposure produces the best looking results. It doesn't directly connect to what exposure saturates the sensor. That is just one factor. For example, less exposure (higher ISO) leaves you with more highlight headroom before you start getting clipping, but the picture will look noisier. Conversely, more exposure (lower ISO) produces a cleaner photograph but your highlights will block up sooner.
One result is that you can get different camera makers deciding on different camera ISOs for the very same sensor, depending on what they think is the optimum balance for image quality. But almost always, they will choose a camera ISO that is somewhat less than the sensor ISO (unless it's a VERY long-dynamic-range sensor) because blown-out highlights are unpopular.
Another consequence is that the camera ISO can change with the camera design, even with the same sensor. The noise in the picture isn't just the result of the sensor characteristics, it's the result of the entire electronic chain, the amplifiers and all the rest. Different electronics designs will produce different amounts of picture noise for the same exposure with the same sensor. That affects the balance between what are acceptable levels of noise and an acceptable range of highlight detail. Lower noise means you can increase the camera ISO and still get good image quality. But the sensor ISO hasn't changed!
That's why DxOMark gives you a plot of camera ISO vs. sensor ISO. It doesn't reveal that a camera manufacturer is “fudging” or “lying.” It tells you how the two different kinds of ISO, which are legitimately determined in very different ways, compare.
That's all. It's not an exposé. It's just technical information. So far as real photography practices go, that particular plot isn't important.
pax \ Ctein
[ Please excuse any word-salad. MacSpeech in training! ]