That's because the white paper only mentions the feed price. Bias is just a convenient way of expressing how much we are deviating from market price.
Everything in the block chain uses the feed price to convert to and from steem and sbd.
I should mention I had this same confusion when comparing the white paper to steemd witness page settings and the bias discussion.
faaaak my brain doesn't wrap around the maths as well as I'd like it to... much thanks to you for your previous work, and now for helping me closer to understanding.
Yay @eonwarped that's our genius and as I can see so many people go to you! You will be a mill before all of us and well deserved too. xoxo