Users use almost enirely German sources.That can be very easily seen whilst one tries to edit articles.

3rd reason:I call it 'thrifty managing of trut'.What it basically means is that Wikipedians of the above mentioned sort 
accept material that 'disproves' the Polishness of western and northern Poland
and delete material that proves it.Deleted without giving any reasons because if they ever gave any they would reveal themselves and their true unwritten agenda.I am a victim of such behaviour.

4th reason: