I've made detailed posts in this thread about this but the X-gene itself is reactionary. It responds to extreme stress, usually life or death situations where the user will die within a short time frame, though some people have a lower threshold. The way Null's X-gene activated all 3 times isn't uncommon.
The X-gene triggers a hyper evolution effect in response to trauma but the final product is MOSTLY, but not entirely, random; If you tortured someone with fire they may gain the ability to control fire or they could develop fireproof skin.
Also, outside of rare cases like Emma, most people can only activate a full X-gene mutation once per lifetime. Children of mutants can inherit their parents mutation (though many don't) and get a second one.
Once a X-gene is active it matures with user and the mutagenic effect remains active for several years, longer for adolescents, then goes dormant for the rest of the users life. That's why many mutants die instead of getting a second mutation to survive a traumatic event, they used up their mulligan, Null got his mutation recently and is young so he's still in the grey area (Ba dum tss).
Cases like Laura where the child gets an almost exact copy of their parents ability is uncommon outside of genetic tampering, which did happen with Laura.
Because he's already on a set path.
A good example is Emma again, she has telepathy but after some stressful events develops telekinesis in some continuities. Emma already had psychic potential, with the right push the ability expands.
Null is the same, his power directly interacts with the energies that mutants give off but not their physical characteristics; Laura's claws don't disappear and Kurt doesn't stop being blue but Laura can't regen and Kurt can't teleport.
All the abilities he develops are interlinked with energy manipulation, his drain power takes energy and his regen power requires his own or stolen energy to work. Even his nullification power is a form of energy manipulation, psychics are good examples, Null can nullify psychic energy trying to get in his mind but even while doing so it doesn't turn off the psychics ability being used at range, it is acting on the energy of the mutant not the body.
Null isn't copying or imitating powers, he says it himself "it took INSPIRATION from".
His current mutation is still in it's infancy and is growing based on the necessary response to trauma and the energy of others his bodies energy field has interfaced with, whether to nullify or drain. He nulls or drains, Rouge, Laura, and Jean 99% of the time, their x-gene signatures are most of the energy he has going through him.
