It looks to me like the real instancing algorithm is a bit off. If it only works properly on 2 out of seven GPU's then I suspect you will have to make a few adjustments for compatibility, some of those cards should show the same distinct improvement that two of them show. It looks like the nVidia drivers are being forced to perform a software emulation on the CPU. It could be something as simple as using an extension that's too new, it can a while for manufacturers to play catch up with one another.
There is an instancing demo available in the PowerVR SDK, available for Windows and Linux, and there is an another example published by Mali as well. Maybe if you compare what you are doing with what they are doing you might be able to get it working on more cards.