Dear, Oldkayaker!
Just my opinion:
Gross Tonnage is the name of web site, which was used by Shipspotting as a source of ship's data. As far as Gross Tonnage had downed, there are many discrepancies in ships' data on Shipspotting.
As was mentioned above: the vessel was initially announced as bulker, and later on changed for tanker, probably due to market conjuncture. Bulkers and tanker are quite similar ships in initial stage of construction. Ship's data was linked from Gross Tonnage as bulker, and froze in this condition. While real data was ammended, as the vessel bocome tanker.
BRGDS
Ruslan