The range of an aircraft depends on the payload. The value you typically find is the range at typical payload.
You can find the payload/range diagram in the Boeing 787 Airplane Characteristics for Airport Planning document:
As you can see, at maximum zero fuel weight, the 787-10 range is only ~4200 NM. According to Wikipedia, the typical passenger configuration for the 787-10 is 330 seats in a 2-class layout. Assuming 195 lbs per passenger (FAA average adult passenger weight in winter) plus 50 lbs baggage per passenger, gives a typical payload of 80,850 lbs. This would result in a range of ~6000 NM. If not everyone onboard is an adult and not everyone takes the full 50 lbs for baggage, the range can be higher of course. Completely empty, the 787-10 can easily fly over 8500 NM.