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Speculation and wrongful design of a currency that was intended to be pegged but actually never was pegged is the answer.
A currency can never be hard pegged unless there exists a mechanism to instantly reduce supply by burning it and to increase supply out of thin air when there is high demand. Both mechanisms are missing with SBD. SBD never was pegged. It was just hovering around one dollar because people believed that it was pegged.

I always understood if you redeemed your SBD you received the equivalent of us$1 of $STEEM after 3 days. Is that no longer the case ?

This is still the case. if you convert 1SBD to Steem via conversion tool, you get 1$/0.35145 (internal median price) = 2.845 Steem currently worth 1$.

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STEEM 0.27
TRX 0.12
JST 0.033
BTC 62120.17
ETH 3179.04
USDT 1.00
SBD 3.75