STP Trading price on Bittrex actually went lower than the tokensale price few hours after its listing. I guess that was week hands letting go and whales buying more. Now, STP tokens is on the rise.
Well, most of the tokens got sold during a private sale. So the private investors there probably got a better deal than the investors in the Bittrex IEO. That's most likely the reason why its trading below the IEO tokensale price.