Its because the price is taken from ripplecharts.com, which averages together the ripple price against many different currencies: BTC, USD, CNY, etc.
The averaging means that when a trade occurs an it moves in one pairing, the average price still doesnt move all that much, so moves occur more smoothly.
Compare to bitshares, where a price move on btc38 makes the price listed on CMC jump around up or down.