Piano Man,
Even if we go with your numbers that's actually only 655kg. 5600lbs/2.2 = 2545kg - 1890kg = 655kg
If that is what your plane's weight and balance says then ya you're legal for 655kg and really we aren't too far apart so I stand by 1100lb / 500kg statement.
This is based on my experience with a company that had a couple of very serious accidents and part of their corrective actions they had all three amphib beavers reweighed by an qualified and independent company. They all gained a couple hundred pounds even though some had been weighed only a couple of years before.
When it comes to Beavers there is always a push to underweigh them to get more usefull load, becuase everyone knows they can haul well over legal gross weight. Maybe you had an exceptionally light beaver but it is also possible that it is just one of the 90% of beavers that are underweighed.