This actually applies to all three programs in the Suite. The color picker in CMYK when used in click-and-drag mode, runs into a problem with its local readout when the first two elements (C,M,) are both 100. When that happens, the field length for the readout does not expand far enough, and the K value is truncated at one or two digits, depending on the number of digits in the Y value. When that happens, the only way to get the accurate K reading is to let go of the mouse, so the full set of readings appears in the palette on the side. But this is quite a nuisance when it happens. I've made the attached screen capture video to demonstrate.
This is in Windows 8.1, using a 1920 x 1200 monitor.
The video is in .MOV format. If it doesn't run properly, I'll upload another one in a different format. Let me know if it's necessary.
AFPhoto.mov