Hi apologies if this has been covered before - but why is there is a price discrepancy on Rialto price when there is only 1 exchange?

Price on cmc is 47.8c
https://coinmarketcap.com/assets/rialto/#markets

Price on ethplorer.io is 30c
https://ethplorer.io/address/0xb24754be79281553dc1adc160ddf5cd9b74361a4

As far as im aware you can only trade Rialto on etherdelta - so why the price discrepancy?

Thanks