Why first person narration is tricky

I just finished St. Lucy's Home for Girls Raised by Wolves, a short story collection by Karen Russell. The stories were inventive, although they're mostly in that inconclusive style that's all the rage these days, which did get tiring after the first couple. But I liked the title story very much indeed and would recommend reading at least that one, if nothing else.

Russell is particularly fond of using first person narration, and frequently from the point of view of a young boy or teen. But I had major cognitive dissonance when she had a 12-year-old narrator describe his father as having an "embraceably lanky physique." Broke my suspenders of disbelief, it did.