The 421/422 train numbers are the "virtual" train with cars that run-through to/from Los Angeles. This means the passengers don't have to change trains - the cars they are occupying are switched to the other train, in this case the Sunset Limited. Jackal's description of the fares and why the prices are different is accurate.

A similar situation happens on the Empire Builder. At Spokane, WA the train splits with one section going to Portland, OR and the other heading to Seattle. The Empire Builder to/from Seattle is trains 7/8, while the Portland section is 27/28. This allows someone boarding in Minneapolis to book non-stop travel to either Portland or Seattle without needing two sets of tickets with different train numbers. The by-product is each train number serves all possible city-pair combinations between Chicago and Spokane as well.

Best advice - book the cheaper ticket when this happens.