because there is no species of phoenix that i know of that is angrier than the generic version and as such have not been distinctly given a morph effect.
and saying momentum+adrenaline as the end result rather than "nixie" isnt valid unless you go to the above comparison with mummy and have it be mummy -> revert from death -> summon scarabs and then say "huh!? why does that have anything to do with the past!?- oh, its because its now a pharaoh, and pharaohs do that" so with pixie you go pixie -> make angry -> become a nixie, oh and nixies are X so it now has X too, because it is now a nixie, not a pixie.
and to distinctly answer your question: the same thing can effect different people/things differently. putting water into oil is different than putting water into pure sodium (boom). you just have to see the connection and why it would be different. so while the rage potion, or water, remains the same, the effect on two different things can be different. thats where the creativity of morph comes in.
and also, keep in mind that ashes dont morph into a phoenix when they are reverse time'd, even tho they could thematically be alongside mummy. but our goal here is to come up with new morph effects, not just hump some more out of reverse time.