It is difficult to explain but I will try. Apophysis works with “mathematical functions” that are called variations.
To make a concrete example or a creation process: I open Apophysis and set a blank canvas. Then I put first layer and put a variation in it that creates a bubble. Then I put second layer with a different variation that multiplies the bubble into bunch of bubbles. Then I put third layer with yet another variation which will bend the bunch of bubbles into a bubbly spiral.
There are over 100 different variations now. It is possible to put more variations in one layer and it is possible to put the same variation in many layers too. Also I have to set a value of each variation to determine its graphical impact and the value can be pretty small like 0.001 or very high like 1000. In some cases, changing a value from 0.001 to 0.002 may completely change the whole image. I can put as many layers as I wish too. All this means that fractal images created in Apophysis are done by the human who sets the variations, not by the program itself.
To sum it up: over 100 variations, the possibility to combine them in one layer and have multiple layers, the vast range of values of the variations – all this equals that each fractal image created in Apophysis is 100% original because it is impossible for 2 fractal artist to create exactly the same combination of variations, values and layers… there are just too many possibilities for that.