Frames are rarely hardened, they usually measure out in the mid to high 20's. Slides don't need to be hardened either, early 1911's were only hardened at the slide stop and the front section to just behind where the back of the spring plug tube is. More modern 1911's they usually harden the whole slide to allow for an even finish.
If your frame is cast, I suspect you measured over a void and got an erroneous low reading. If it measures 12 all over, sheesh.