Sounds as though they were carrying a fuel load [that is a MUST], that limited their take-off weight, or perhaps single engine enroute altitude capability. Fuel loads are dependant on many factors, least of which is not departure, destination, alternate, and enroute weather [how was Brussells weather?], and additionally, Air traffic Control [ATC] holding requirements.
Even though there were 2 spare seats, the aeroplane would still have been at maximum take-off, and/or landing weight.