The more expensive L and U is a broken fare. You are essentially purchasing two different tickets written as one, which is more expensive that purchasing a normal ticket with a through fare. This is another example where you would expect L to be more expensive than U, but doing the trip partially in U makes it much more expensive.

Unless you need different fares on other segments of your itinerary, what usually works is to specify in search by fare class the more expensive fare class (L in this case) that you see to get rid of the broken fare and lower the price.