youre right the stubs should be very different because they are! 04 sti axles are female which uses a stub and 06 wrx axles are male which dont use a stub. and the male wrx axles are smaller in diameter which makes a smaller inside diameter axle seal is needed. So you would've needed to change the axles seals at some point. theres only 2 seals, one that is meant for female axles (larger inside diameter) and one that is meant for male axles (smaller inside diameter). obviously you used your male wrx axles at first and you had a leaking problem and then you put in the female 04sti axles and stubs and that solved the leaking problem. That proves that you had the female axle seals in your tranny which are the wrong ones for male axles hence the leaking problem.