[mesmark]

stanley - 'how come' may be an American thing. I use it quite frequently but it is definitely very casual. I don't ever use it within a sentence, that I'm aware of, that's why it struck me as odd.

I use it a lot stand alone. - 'How come?' For that reason I teach it to my high school students. It seems to come up a lot when I'm trying to be cool and pal around with the kids.

It's actually a book produced by Japanese authors and they're trying to teach 'real' English in that text. Sometimes they stretch things a little too far. Like you said, it's not really something they 'should' know, especially at their level. (A good deal of the time, they can't even use 'why' correctly.)