Actually, the number you quote is for double occupancy of each cabin. Freedom can handle 4370 passengers if all the 3rd and 4th berths are filled.
Cruise ships often sail at or near capacity. Why build a ship that can hold 4370 people and not try to fill it? Whether a cruise sails at capacity depends on many factors - not the least of which is the time of year and the popularity of the itinerary. It is quite common for Caribbean cruises in the summer to be maxed out. The same cruise probably won't be full in the fall or early December.
We typically take cruises in the summer and they are usually full ships. On the maiden Freedom voyage we took in June, RCCL made a decision to not sell the ship out. They wanted to get a handle on how the ship would be with all of those people so we sailed at a maximum capacity for the Voyager class - which meant we were several hundred passengers short of a full load. RCCL then added more passengers as each week went by. I imagine Freedom is often leaving Miami with over 4000 passengers now.
The is no "normal". If a ship is properly designed it will handle a capacity load of passengers without making things feel crowded.