It is pretty clear once you look at the code.
// This block limits the effective median price to force SBD to remain at or
// below 10% of the combined market cap of STEEM and SBD.
//
// For example, if we have 500 STEEM and 100 SBD, the price is limited to
// 900 SBD / 500 STEEM which works out to be $1.80. At this price, 500 Steem
// would be valued at 500 * $1.80 = $900. 100 SBD is by definition always $100,
// so the combined market cap is $900 + $100 = $1000.
XD