I think it all comes down to individual driver style. Riccardo has been driving a sub-par car for the last few years and had to push it to get the results he achieved. Vettel has had a superior car for the last few years and was able to take advantage of that. These new rules have played havoc with Vettel's driving style. The back of the Red Bull is not planted and that doesn't fit the way he is used to driving. Thus his teamate is adapting better and is faster.
Fred has always pushed no matter which car he has been in. He is good at giving excellent feedback to the engineers to make the car better. I don't think Kimi is quite as good as Fred at giving feedback. Kimi also is uncomfortable with the Ferrari's brake by wire system.
Raikkonen: brake-by-wire system at root of early Ferrari F1 issues - F1 news - AUTOSPORT.com