By comparison, Qantas Airways expects the maximum Project Sunrise flight time with the new Airbus A350-1000ULR to be 21 hours and 40 minutes during commercial flights between Sydney and New York or London—more than enough to set a new world record. The latest test flight certainly surpassed that benchmark.

I am guessing that a test with minimal self loading cargo will need to go further. With <some tons> more cargo it will go <predictably shorter distance>

Or in other words, it went further than the rating because it was unloaded.

Just a guess, mostly to provoke the real aerospace engineers into commenting.