Above explanation pretty much sums it up. Function of strength vs. capacity. The calculators you find online simply correlate the surface area of silicone on the tank wall to the load (in this case, water pressure) being exerted on it. Each type of silicone will have a different value for adhesion strength, usually in N/mm^2, so the safety factor will always vary somewhat according to the type of silicone used, and will generally vary much more depending on surface area (thicker glass=stronger bond).
I see the term brought up a lot in tank building discussions, but I think that too often the concept is misunderstood and not applied correctly in practice. Even with a safety factor of let's say 4, a lot of tanks will still fail when/if experiencing shock load, like if your 225g tank made with 1/2" glass were to shift suddenly and unexpectedly, the resultant tsunami in the tank could cause a blowout. I'm not saying that drunken idiots are bound to pass out and stumble into expensive aquarium sets every day, but hey this example is needlessly detailed for a reason.
