Anyone else, please correct me if I'm wrong, but I believe the discrepancy is based on the way the STEEM blockchain handles its valuations/exchange rates. It works based on a seven day rolling average, so when you signed up, the $10 was awarded to you based on the current exchange rate, but your $3.18 in your wallet is a product of priceAverage * steemHeld = 3.18

Hopefully that helps! Welcome to the party!