If one did want to make directly translate between the two systems, you'd actually find out that the ego functions for extroverts correspond to the dominant+auxiliary functions in MBTI, while this is not the case for introverts.
It's not that the conversions are inaccurate. It's that no one bothers to actually read about socionics cognitive functions, and just assumes that when it says Te on socionics.com, it'd have to be the exact same thing. (The little symbols - triangles, circles, squares - that they use are intended to prevent confusion.)
You'll notice that in Socionics, INTJs have Ti & Ne for their ego functions. And Ni & Te ends up being the id block of functions, where Ni is supposedly as strong as the leading function, but not consciously experienced.
I think sections 4 & 5 from the wikipedia page explains it well:
Socionics - Wikipedia, the free encyclopedia