Because for some reason it was decided that the last letter at the end of each type would be for what function that type extraverts in, so INFP's for example would extravert Ne as that is the function they most outwardly use extraverted types of course use their dominant.
I might have to check Gifts Differing again, but I can't remember a decent reason off the top of my head for why it was structured that way.
Essentially, though, it appears to be something to do with the nature of extraversion and introversion. So introverts don't openly show their dominant, instead their secondary comes to light, whereas with extraverts their dominant is extraverted already, but people don't always notice their capacity for introspection because of this.
And since extraversion has more focus on the external, it follows, (on the creator's terms of course), that IP's are perceivers, because that is how they relate to the outside world and it's structure and the same goes for IJ's as judgers.