The exchange rates vary. They may be using something else to price the steem and likewise the bitcoin may be priced differently... For example I use bitstamp.net and many others may use something else and there are some differences.
Check Coinmarketcap.com and do the math and see if it gives you a similar number or the 166 steem for 1 btc?
I get $653/$3.71 = 176 Steem according to coinmarketcap.com
I think its close enough, again depends on what exchanges they are pricing steem and btc.
Wouldn't they go by what the price is on their buy and sell page? Right now it's at 3.40. 2 days ago steem was at 2.97 on their page and Bitcoin was like $666 and it was well over $100 usd difference.