Yes the file was CMYK/8.
I've been putzing around and have come up with:
File colour profile was Coated GraCol 2006 (ISO 12647-2-2004) and if I change the profile to match that in Acrobat Pro then I get my real 100% black with CMY reading 0% as it should. If I spec the file as US Sheetfed Coated v2 then in Acrobat with the same profile I get 72%C, 66%M, 66%Y, 78%K. I spent a couple of decades in Prepress in a large, up-to-date Printshop and we used the Gracol profile which worked best with our computerized platesetter. Not sure why this is happening, hopefully the developer guys will come up with a solution. Seems like a colour profile issue. Maybe we need an "accurate black" choice in Publisher preferences similar to InDesign.
trial (Coated GraCol).pdf
trial (US Sheetfed).pdf