Your imatest numbers are definitely less than mine for testing similar backs (p20, CF 22, CF 528, CF 39, etc) however I can't say why yours are lower and I haven't run your files through imatest myself. In my own testing, the imatest low values come closer to DXO's values and also fit with the sensor manufacturers stated DR. But does DXO test camera and lenses or just the sensor? Lenses may limit contrast and therefore DR - so they may have some effect. In fact as a side discussion, I wonder what the DR limits of lenses are? I've read somewhere that the most we can expect through good real life lenses is about 13-14 stops. Anyone have any info on that? Certainly we are at the point where the lenses can make the difference.
Since I have tested many cameras, the thing I find most useful is the ratio of drop from low to high values in imatest. DSLR's of past used to drop quite a lot from low to high - ie about 4-5 stops. Current generation DSLR's like the Nikon d800 don't drop as much between the ratios. I find the Imatest High value of most use to evaluate how an image will hold up through the editing process.