My memory is that HNL-GUM and GUM-HNL travel is unique in that you are not technically processing through immigration (so US Citizens do not need passports), but because Guam is considered “outside the United States Customs Zone” you process through Customs.
With that said...it has been a few years since I have made that trip and things may have changed.
This is as good an explaination as I have come up with. Just went through this on Sunday and it is odd. You will go through Immigration in Guam (without your bag) and customs in HNL. I gave up trying to figure this out.