I would say "how so?" means more like, "In what way?" or "give me some examples." "How come?" is basically a general way to ask why something happened. I don't think she's a very good teacher. "How so?" - explain to me why she's a bad teacher. "How come?" - why do you THINK that? She dressed very elegantly. "How so?" - what was she wearing that made her elegant? "How come?" - WHY did she dress elegantly? Hope this helped!

'How come'- Is a phrase of come. It is a informal way to ask how or why something happened or is the case. 'How so'- A phrase one uses when he or she wants the other to explain.

It's a tough one to think about. They are both more informal than using 'why' and get the same message across. They also have the same meaning (in what way/why?), but 'how so' is used a lot less often (At least in American dialect). I think you will find that people use the two interchangeably. Hope this helps, Jaron