I bet that many of shapeshift.io's *precise* conversions fail, because they only give them 10 minutes to complete (and sending can take longer than that). Then, shapeshift refunds whatever you sent, but it's minus a miner fee. So, if they charged a slightly higher miner fee than what they actually had to pay the network to get the refund to you, they could profit on volume, without having to actually do any conversion.