Thanks to liushie, a math domain error was caught in the CIE1994 and CMCcolor difference methods. This only manifested itself for certain CIELab values, and went unnoticed as a result of this. The problems have been corrected and released as 1.0.6.
Please see the project page for more details on what python-colormath, how to install it, and how to use it. This is unlikely to interest you if you are not looking to do some kind of color math.