Firstly, I would suggest going and reading this answer. It's actually for the exact opposite question to what you're asking, but the background in the answer will leave you clued up to understand what's happening.
In this case, if you look at the fare that's being used when you select the one-way AKL-LAX flight (fare class OLOWNZ) it includes the rules :
ROUND TRIPS/CIRCLE TRIPS NOT PERMITTED.
END-ON-END NOT PERMITTED.
Thus this fare can only be used when booking a one-way trip. You can't use it for a "round trip", or a "circle trip" (think a triangle with 3 cities).
If you look at the fare being used for the more expensive round-trip option (fare class OLEEUS) the fare rules include :
CIRCLE TRIPS NOT PERMITTED.
END-ON-END NOT PERMITTED. SIDE TRIPS PERMITTED.
However there is no mention of round-trips, so this fare can be used for a round-trip. (It could also be used for a one-way trip, but given there's a cheaper fare available it generally would not be).
Whilst it's common for an airline to have round-trip only fares (fares that can not be used for one-way flights) as a way of making round-trips cheaper than two one-way flights, it's uncommon to see a flight marked as one-way only as the first fare here is.
It's possible that this is a mistake, however given that the OLOWNZ fare explicitly disallows return trips I'd say that's unlikely. More likely the airline has a specific reason for offering the flights at these prices, with the expectation that most people would simply book the more expensive return flight. I wouldn't even like to guess at what those reasons are...
As was mentioned in a comment, the 'sales city' can also play a part when searching like this - although in this case you've set it to be the same for each search. Airlines can and do change prices based on the country where the ticket is purchased (often referred to as the 'point of sale'). By default, Matrix uses the origin city as the point-of-sale, so for your return flight option it would use the US (LAX) for both legs, but for the two-one-way option it would use the US for the first, but New Zealand (AKL) for the second. If the NZ prices were significantly cheaper, that could also lead to something like what you're seeing - however it's not the case here due to you forcing the 'sales city' field in Matrix to be the same for all searches.