Realizing that this is the Basic Scuba forum, I thought I would explain some of the basic ideas here that are being bandied about as if every reader was already an expert.
When a diver exits the water and begins a surface interval, the diver's tissues contain more dissolved nitrogen than is in the air they are breathing. When they breathe, nitrogen in the air enters their tissues, but nitrogen in the tissues leaves into the air. Since there is more nitrogen in the tissues than in the air, the net result is a loss of tissue nitrogen--off-gassing. (More nitrogen is leaving the body than is entering it.) The greater the difference (gradient) between the tissues and the air, the faster the offgassing occurs. As the gradient becomes less and less, the rate of off-gassing slows down.
Different tissues also absorb and release nitrogen at different rates, depending upon the physical nature and their blood flow. Very fast tissues will lose their nitrogen quickly, and very slow tissues will take much longer. To estimate this off-gassing, the body is said to have a variety of faster and slower theoretical tissues, called "compartments." Each theoretical compartment is identified by the rate at which loses gas (and slows down that loss over time). Those rates are called "half-times." The fastest compartment is called the 5 minute compartment. That means that when the diver surfaces, the gradient between that compartment and the air will be cut in half in 5 minutes. The rate at which it slows down is also calculated by half-times. In the next 5 minutes, it will lose half of what is left. In the next 5 minutes, it will lose half of what is left. In a total of 6 half-times, it will be effectively equal to the air.
It is important to lose enough nitrogen before the next dive to be able to start that dive at a reasonably safe level. To do that, algorithms have to identify the compartment to use to guide the length of the surface interval. The fastest ones are discounted because they lose their excess gas quickly, and the slowest ones are discounted because they take on gas so slowly that they do not have enough to worry about. The Navy decided that the 120 minute compartment would guide surface intervals, and PADI decided that the 60 minute compartment would guide surface intervals for recreational divers. That is the reason for the difference between those tables. When recreational divers push the envelope of their dives, the 60 minute compartment becomes less suitable, and a longer surface interval is appropriate.
