True .. but there is no rational reason (at least to me) that the bitusd is worth LESS than the usd... any explanation?
I think there are a number of ways to look at it, but my favorite is this: the price of BitUSD is a combination of the price of USD and the short-term expectation of the price movement of BTSX. If people think there's a good chance the price of BTSX will rise quickly in the short-term, they'd rather short bitUSD than buy it. This is rational! If bitUSD is trading at a 20% discount but you think BTSX will double in value in the next two days (like it did earlier in the week), your expected profit by shorting bitUSD is
more than your expected profit by buying bitUSD and supporting the peg. It's something like a tragedy of the commons.
This could be a problem - it means that when BTSX is low-volatility, we should expect to see a tighter peg, but that volatility in BTSX should cause fluctuations in the peg.