I believe you've mixed lists. "Ordinary" doesn't refer to an emulsion speed, but rather to the plate being colorblind, i.e. unsensitized to anything but UV and a bit of violet light. It is by default very slow, but an ordinary plate can still be different degrees of slow.
It's been my understanding for so long that I've forgotten where I learned it (and could therefore be wrong!) that the X designation referenced chemists' notation for halides (in photography: chloride, bromide, and iodide). AgX is shorthand for any silver halide.