The cameras all expose accurately! Your images will not be under exposed.
DXO is confusing the issue with their measurements, which lead some to think there images are not correctly exposed. You can find DXO's explanation saying this buried in their test methods, but the bottom line is that the ISO's are accurate and DXO is measuring the sensor and not accuracy of the camera at each ISO setting. They are known for inventing their own tests and then claiming they mean something while confusing people in the process.
DPR, for example actually checked the 70D camera and found it to be within 1/6 stop.
I wasn't trying to claim that actual exposure was off by this amount, but I take your point: if the exposure is correct and the SNR is acceptable, what the heck does it matter if DXO Mark's measured ISO is different to what the camera reports. Re-reading DXO's tests, I think that the relevance of the DXO method based upon measuring the exposure required to reach sensor saturation, is to allow us to understand what decisions the manufacturer has made about how the sensor should handle the signal.
Given that actual sensitivity of the sensor to light is fixed, I assume that what they're measuring is the amount of 'hardware' amplification that the signal is given. I have seen it written (I am no expert) that all cameras use hardware amplification up to a certain level and then start to boost the signal using 'software' methods. Thus the difference between measured and manufacturers' ISO ratings must be the amount of 'software' amplification after the signal has been converted from analogue to digital. I would assume that software amplification would generate greater noise than hardware amplification (or why bother with the latter?), but again this is beyond my level of expertise. I'm guessing that under amplifying the signal from the sensor allows Olympus to retain more of the highlight data, perhaps at the expense of shadow noise. If Olympus are using one of Sony's excellent current generation sensors, read noise might not be a problem. This would therefore enable them to make the output from their cameras more forgiving in the highlights.
As I have stated previously, this is all speculation on my part as I am no expert in this field. Perhaps you are right when you criticize DXO Mark for presenting this data without more extensive explanation of its implications, thus opening the door for DXO data to be used as fuel for unwarranted claims. Perhaps it is us (well, me
in this instance) that are the real problem; a little knowledge is truly a dangerous thing!