The joint-venture has more to do with setting flight schedules, revenue sharing for TATL segments and making seats available to one another via codeshare. It is a collaboration among competitors, but one of the things they can't do is collude on setting fares, as that would be easily shown to be anti-competitive and regulators would be all over that and shut the joint-venture down. While it is acceptable for competing businesses to collaborate in an effort to seek profit, they are restricted on what they can do to prevent collaboration from turning into anticompetitive harm to the consumer. It would be very concerning, as a consumer, if their fares were all in-line with each other, as it would suggest there was actual collusion going on to set fares, and would then raise concern that they could be using their collaboration in order to drive fares up higher then what the market naturally settles to.

As to why there may be such a large discrepancy in price, I would say it is no different than fare differences you would see from other airlines flying between the same two cities. These flights and fares are not the be-all end-all for money allocation for this specific flight, but rather the fare needs to be looked at in the larger picture of the airlines operations as a whole. Both LH and UA have two entirely separate networks on both ends of the Atlantic which are not part of their JV, and the TATL flight is only one piece of a fare that may go to city pairs completely different from the actual destination of one specific flight. The airlines need to consider the impacts both the O/D traffic and any connecting traffic that might want to fly on a specific flight. Airlines care about and try to control which flights their O/D traffic and connecting traffic end up on (i.e. very simplified example - I have 2 flights to BBB, one arrives at 9am, the other at 12pm. If destination BBB has flights to 50 different cities that all leave before 11am, and nothing the rest of the day, then I don't want my 9am flight to BBB being all O/D traffic, and my 12pm flight being all connecting traffic). One way airlines can drive where passengers are booking is by their pricing. Sure, you can still buy a seat on the 9am arrival, but now I risk not having a seat for someone who wants to connect beyond BBB, and now they might not fly with me at all because a competitor can get them to their final destination too, so I'm going to charge you more for taking that seat on the 9am. And again, since LH and UA have two different markets not part of the JV, they may have individual reasons for wanting to drive traffic towards certain flights and therefore charge different prices for the same physical flight.

TLDR: the JV only applies to TATL. UA and LH both serve different markets on both ends of the atlantic, and file their fares (and pricing) based on how those specific flights fit in their entire network.