To calculate the dollar value in your wallet, it takes the number of Steem and Steem Power and multiplies them NOT by the current market price of Steem but rather by the AVERAGE market price of Steem over the last 7 days, which is currently about $1. There are a variety of reasons for this.

So, when markets are rising over the last seven days, the REAL value of your steem and steem power exceeds the amount in your wallet. And, when markets fall over the last seven days, the real value of your steem and steem power will be less than what's shown in your wallet.

Additionally, the dollar values that you seen for each post are also based on the average price of Steem over the last seven days and NOT the current market price. If it were based on the current market price the post value would constantly be bouncing around form minute to minute, which would be very confusing to people.