Calculating molar solubility of calcium hydroxide from solubility product
A cursory search suggests the solubilities in g/100 mL on the wikipedia are closer to a consensus, for what it's worth:


solubility 0.2% ($0^oC$): 
https://www.cdc.gov/niosh/npg/npgd0092.html 
solubility 0.159 g/100 mL
(0.16% by weight) ($25^oC$): 
https://www.mdpi.com/1996-1944/10/10/1219/pdf
the value you reported from the National Lime association.


These are reasonably consistent with the values in

https://en.wikipedia.org/wiki/Solubility_table#C

but not with the $K_{sp}=5.5 \times 10^{-6} M^3$ on the wiki page.

Skoog, West and Holler's Analytical Chemistry textbook (7th Edition) reports $K_{sp} = 6.5 \times 10^{-6} M^3$ at 25$^oC$. They provide a ref: 


  most of the data were taken from A.E. Martell and R. M. Smith, Critical Stability Constants, Vol. 3-6.


Note the apparent inconsistency recurs (I called this discrepancy an "error" before but am no longer so sure):

http://www4.ncsu.edu/~franzen/public_html/CH201/data/Solubility_Product_Constants.pdf
http://www5.csudh.edu/oliver/chemdata/data-ksp.htm

It reminds you that you should generally reference your data sources if you want to be helpful to fellow researchers.

The value for $K_{sp}$ in the wikipedia (which is not referenced as far as I could tell) and in a highly respected textbook which references its source is smaller by roughly an order of magnitude compared to values derived using standard equations from reported solubilities in a number of other sources noted above. This inconsistency may be because some of the reported data is for hydrates with higher Mw. Not sure how you could explain such a discrepancy otherwise. If the $K_{sp}$ were much smaller you could argue that the dissociation constant of water affected the solubility.  

My current guess is that there is a reasonable explanation for the discrepancy (ie it has a chemical or physical explanation and is not due to human error). It is unfortunate that the solution is not more obvious given that $\ce{Ca(OH)2}$ is a recurrent example in problems (textbook calculations or experiments) teaching solubility to undergraduates. 

See also:

Solubility of calcium hydroxide determined by titration