I guess, I’ve forgotten to add the territorial water zone (22 km) – so that could explain the difference to the figures from CIA World FactBook
Thank you very much for the info about LSIB.
I tried the Data from Natural Earth 1:10 Mio and LSIB now to calculate the area:
LSIB: ~ 458 000 km²
NEA: ~ 458 300 km²
But several other sources like the CIA World Factbook say something about 527 968 km² or 537.000 km²
The German Auswärtiges Amt says, that the area is currently not known exactly but guessed between 478 000 und 533 000
I try to find out, whether those figures in CIA WF (et al.) are wrong or the boundaries in the GIS data sets are wrong. At some point there must be a larger error.