Great question — answering as the UphillTech team:
It's hard to understand why this can be the case without some background in how airline fares work, so lets start with a bit of background... For each route, airlines will publish a number of different fares, each with a distinct "fare code" - normally with weird names like "TANRKL", "YSJWNN" or "X34Q". Corresponding to these fare codes are "fare rules" which dictate things like whether changes/refunds are allowed on that fare class (and what fees are payable for them), whether stopovers are allowed, etc. In addition to the customer-facing conditions the fare rules include controls over what type of trips that fare can be used for, with terms such as "One-way", "Return", "Open-jaw",...
Full answer by Doc: https://travel.stackexchange.com/a/11504 (CC BY-SA 3.0).
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