Are you referring to the estimated value in your Steemit wallet? If so it's an estimate because the market price of steem is constantly fluctuating, so they give you an estimate of what your steem/steem power/ steem dollars are worth. See this post for more: https://steemit.com/steemit-help/@nanzo-scoop/estimated-value-in-your-wallet-is-just-an-estimate