Hung Hom To Lo Wu (KCRC)

這是因為較多乘客由市區往羅湖過境，九鐵做出的「掠水行為」。
九鐵一直是以過境線（羅湖至其他站）補貼本地線（上水至尖東），所以出現這情況。
如果覺得「唔 gur」嘅，可以在上水站出閘，然後等兩分鐘再入閘，你就可以慳番 $4 ($31.3-$18.8-$8.5)，不過要等多一班車lor！

I think you're asking why the sum of two trips are cheaper than that of a single trip, instead of why the journeys to Lo Wu are much more expensive.

Anyway, as the answer described above, trips to Lo Wu are especially expensive because such cross-boundary service is a monopoly business.

However, at the same time, such profit is compensating the costs of local transportation, thus local journeys are much cheaper than actual price. Thus, the fare from Hung Hom to Lo Wu are fully cross-boundary, and the fare from Sheung Shui to Lo Wu is cross-boundary, but the journey from Hung Hom to Sheung Shui is local! Since the trip from Hung Hom to Sheung Shui is especailly cheap, the total fare is cheaper than a single journey from Hung Hom to Lo Wu.