Many fare filings for international routes are different depending on point of sale (while fare filings on domestic routes are bi-directional). If you buy a roundtrip ex-US, both outbound and return fares will actually use US point of sale fare filings. If you buy one-way in each direction, only outbound fare will be US fare filing, while return will use fare filings from other country. However, there can also be cheaper fare filings that require a roundtrip purchase (these generally also have minimum stay requirement like a Saturday night). So even if fares from destination may be cheaper, overall it may still be cheaper to purchase a roundtrip ex-US than one-way's in each direction. In this case, fare filings from reverse direction look to be cheaper and there were no ex-US roundtrip fares for less (or OP did not meet minimum stay requirements to qualify for cheaper roundtrip fares).

Checking on ExpertFlyer, EX-US fares for RLP00SOL roundtrip (each-way) and RLP0ISOS one-way PE fares on SFO-ICN/ICN-SFO look to be identical $1100 base USD (which is a bit odd, for coach fares on route there are some cheaper roundtrip fare offerings than one-way fares). While ex-KE, a RLXUISDR PE one-way fare for ICN-SFO/SFO-ICN is $858 base USD.