This is not that unusual. Basic Economy fares tend to have more restrictions on fare combinations than the corresponding main cabin fares. If you actually click through on the fare options, you will see that the cheaper main cabin fare involves a broken fare combination on the return flights. The outbound ATL-LAX-HNL fare is a single XEWNLXML fare, but the cheaper return flights involve a broken fare -- KA0NHOMQ on HNL-LAX and VEVNA0MB for LAX-ATL. There are BE fares corresponding to those main cabin fares, but those BE fares are more restrictive when it comes to END-ON-END combinations, while the main cabin fares are less so. Here's the relevant fare combination sections --

VEVNA0MB main cabin fare
END-ON-END COMBINATIONS PERMITTED

VEVNA0BA basic economy fare
END ON END MUST BE AN A-B-A COMBINATION

The A-B-A business means the BE fare can only be booked as ATL-LAX-ATL (A-B-A) and can't be combined with another fare to form a routing like HNL-LAX-ATL (which is not A-B-A routing).

So the only valid BE fares are single fare options with XEWNLXBQ for ATL-LAX-HNL and TEWNHXBQ for the HNL-LAX-ATL return. The TWENHXBQ BE fare is more expensive than the KA0NHOMQ and VEVNA0MB main cabin fare combo which leads to the total ticket cost being somewhat more.