This is pretty common on a lot of UA flights these days, on specific city pairs. IAH-NRT is another, where NH flights often appear to be cheaper than UA.
What's actually going on (in most of the cases I've seen) is that UA publishes fares that approximately match their partners. But then they don't make seats available in the fare buckets corresponding to those prices, while the partner flights do have availability. The NH or LH flights will have P or Z available, while UA metal only has J/C/D.
Last December, UA changed its rev. mgmt. strategy to hold back discount buckets in the hopes of capturing higher-margin fares closer to departure date. I haven't seen any post-analysis on whether this turned out to be a good move or not. But anecdotally, partners are destroying UA's fares for advance sales on some Int'l routes, because of bucket availability!