The mm in @teamsmooth-mm means "market maker." This is a traditional role in order financial markets, now usually performed by large financial entities with sophisticated trading algorithms.
The role of the market maker is to ensure that orderly markets exist. The software automatically adjusts the bid and ask limit orders so as to keep a balanced book. As you pointed out, the Steem and SBD held are very similar amounts. This means that the software is working.

Compare that 0.55% spread to the spread on bittrex... And ask yourself, why should Steem/SBD have a lower spread on the low volume internal market compared to the spread on Steem/BTC on bittrex...

You can trade between Steem and SBD with very low spread because of the service the market maker performs. And as you pointed out, it can be very lucrative.