Is the mean of visible color in CIELab color space can be non visible color?
You are implicitly using sRGB, a small color space. You might like to try using scRGB, a similar color space that allows such values. 

The average of visible colors should also be visible, however the calculation should be performed in a perceptual color space for the result to be more meaningful; L*a*b* is better than sRGB.

Another question to ponder is whether the centers make sense in L*a*b*. Perhaps you have a bug?