Likely what's most confusing here is the idea of "associated with the calcium ions".

What's important to know is that the amount of dissolved hydroxide is a result of the sum of pre-existing and added (dissolve) hydroxide. Since volume remains constant, amount is directly proportional to concentration, which suggests that concentration is additive (added + initial = final)

That, coupled with the ideas that a salt / ionic compound can only dissolve in stoichiometric ratios and that Ksp is the product of component ions raised to their stoichiometric ratios, give you the Ksp of calcium hydroxide.