...Because the Sittingbourne-Charlton fare is more expensive than the reverse.
This is fairly common on commuter flows, since the peak demand is in one direction only, so a premium is paid to go this way. Often there is no difference in Off Peak fares, as there's less difference in demand off peak.
Note that it's not permitted to use the return portion before the outward (you can bin the outward and only use the return though).