The optimum IAS for gliding range (ie max L/D speed) is slower than the suggested Cruise IAS, as I stated before, depending on weight.
Though I cannot speak for Cessna, the most likely reason for their recommended 'Cruise IAS' being higher would be that 'everyone wants to get there', despite the optimum range being compromised. Small changes of speed around an optimum make negligible difference, so higher speeds will be more economical of time, without being too detrimental to range.