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Because the value of the tokens is determined by the exchange rate of STEEM, thus if steem is rising or falling on the exchanges then the value of a post will also rise and fall depending on how much steem the post has earned.

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STEEM 0.18
TRX 0.15
JST 0.029
BTC 61733.68
ETH 2481.63
USDT 1.00
SBD 2.63