There really is no such thing as true infinity with all M lenses in actual use, unless you send all of your M lenses in to be calibrated, which is why most of the adapter makers leave some tolerance. For example, my first Hawk's adapter nailed infinity with a few of my lenses, and it didn't quite reach infinity with others. My newer, version II Hawk's adapter, as well as my Metabones and Voigtlander, focus past infinity with most of my lenses, and nail it with one or two of them.
Erwin Puts says that Zeiss calibrates their lenses to the front plane of the film, and Leica to the back, and, the potential of that combined with the varying differences in tolerances forces most adapter makers to leave a little slack so that infinity is guaranteed, at the potential cost of a little mfd lengthening.