generally its a random thing. but now there also is a recessive morph that does it. atomic completely covers the whole BP with lots of paradoxes.
http://www.worldofballpythons.com/mo...ic-fire-sugar/
http://www.worldofballpythons.com/mo...mic-queen-bee/
its still a very rare and very expensive morph, and the basic version doesnt look impressive, you need to add a few morphs, and then you see the different morphs being expressed in different places.
but i agree, if its just an ivory, you can be quite certain that its a random thing and not genetic.