I read that the composition of Bleaching Powder was actually a dispute going on for decades.
One opinion, confirmed by experimental measurements of available Chlorine, was that the best formula was CaCl(OCl), a chloro-hypochlorite mixed salt. However, subsequent studies using X-ray diffractions found no evident supporting this structure.
What was found a mixture of Ca(OCl)2 and several hydrates of CaCl2.xH2O and Ca(OH)2. These structures support the problematic observation that the Bleaching Powder does not have an affinity for water like dry CaCl2, as the bound CaCl2 is apparently already in hydrated form!