It may just have been bad timing. China’s announcement of a looser renminbi peg came only a few days after a downbeat report on exports. So when the renminbi news hit, there was a common shocked response: China’s economy must be much weaker than we thought.
This reading took hold despite the availability of another explanation: China is keen that the IMF add the renminbi to the list of reserve currencies. This requires that it be more market-driven; the loosening was a step in that direction.
Why is it even possible to regard the peg as an economic indicator? The conventional answer is simple and cynical: you cannot trust the official economic numbers, so you have little choice but to read into official pronouncements and actions.