I don't do Guam travel, but I do know that the customs duty/tariff due rules for goods flying from outside of the US into Guam and terminating in GUM are different than if flying to the US via Guam from beyond the US and US possessions/territories or if flying to the US merely from Guam.

That the person is flying to HNL rather than merely to Guam means that the potential duties due are different. That the person may acquire things in Guam and brings it to the US 50 that are subject to US duty means that the U.S. CBP actually may need to calculate things differently based on the GUM-HNL flight number than on the ICN-GUM flight number.

Raehl311's way of thinking of what to do/leave makes sense, as it sounds like the CBP auto populated at least from a flight booked by/for the OP. [My situation type shared in my prior post have been more complex and are distinct from what the OP experienced, so ignore that post.]