They don't need to use the same sensor. They have a sensitivity to light based on the amplification in the electronics. That sensitivity is calibrated against standards produced by the ISO, hence the reason we lazy people call it ISO. My point about DxOMark is that they test cameras against the standard, and plot the accuracy of the manufacturer claims across the range of possible settings. There's relatively good consistency across all of the cameras I've perused, with the caveat that a lot of Nikon DSLRs don't natively do ISO 100 (it's a software hack; shoot at ISO 200 then mathematically decrease the brightness).