technically any charterer can advise their agent to not list...

technically any charterer can advise their agent to not list their cargo details on the port website. maybe the others are happy to broadcast this information 

why they would/would not? that's a different question. Most likely because next vessel is next month as advised by the happy chappy that called the port authority

But yes, if GXY wants they can tell their agent to make sure the Port Authority doesn't publicly list their details. Its been noted in the past especially up in the pilbarra ports when iron ore comps would not want to broadcast their intentions for bringing in rail lengths and rollingstock equipment; so those berth line-ups would say something like 'general cargo' or 'steel beams' when in fact it was locomotives and rail lengths. Other shipping agents hated it because they'd attempt to calculate when their vessel would come alongside but specialized cargo like locomotives had discharge restrictions, so you might see 2000mt gen cargo (that you'd estimate be discharged in 36hrs)  but in fact it would be 10 locomotives that can take 3 days due to day light restrictions and delivery site restrictions etc etc. anyways just a shaggy dog story for you all