The Cabin OK guideline is smaller than the size set by most airlines as their maximum acceptable for carry-on baggage. Thus, passengers with Cabin OK carry-on baggage can travel with a greater assurance that it will be acceptable across the different airline requirements. And, when travelling on a participating airline there is a further benefit: those bags with a Cabin OK logo will have a priority (determined individually by each airline) for staying in the cabin should its cabin capacity be exceeded and some baggage need to be moved to the hold.
What they're trying to say is the following: thanks in part to airlines charging for luggage, passengers often encounter situations where the plane is full and some bags are gate-checked, at no additional cost to the passenger. On some of the smaller aircraft, many "perfectly legal"-sized bags are out of necessity gate-checked. The "Cabin OK" logo is IATA's way to signal that, barring exceptional circumstances, that bag need never be checked at the gate. The idea is that the gate agent need only grab the trolleys without the logo to ensure space on a full flight.