The reason be is that It must be considered critically from the viewpoint of those who wrote it. If you are going on the Polish and or German historians they are going to write it as how they see foot. Some information is going to possibly be false from either side. It is hard for many historians to see eye to eye and thus you will have conflicting reports.
Europe is rarely centralized and still faces problems along ethnic and regional lines. Concerns of citizens in Castillian Spain are far different from those in Scotland, Italy Poland. Likewise, research into Polish and German historians regarding the Bromberg Bloody Sunday incident will have similar biases. Historians rarely see eye-to-eye and different ethnic groups and nationalities will color the facts of those writing history books.
Although the most widely held view of the Bromberg Bloody Sunday proports that the murder of ethnic Germans in the city of Bromberg was the result of a German attack against Polish military troops, there is some belief that because of the Communist-Polish recording of history, the causes of the attack may have been falsely blamed on a German insurgency, but in actuality was the result of panic mistrust of German citizens by Polish military after the Invasion of Poland began. It is for these reasons that the version of events related to the Bromberg Bloody Sunday incident can be perceived with bias by Polish and German Historians towards their respective nationalities.
Yes, I think that the Bromberg Bloody Sunday research of Polish and German historians is inherently biased because of their respective nationalities, because the research has to be viewed from the perspective of those who are writing. The writing cannot be viewed in a vacuum. It must be considered critically from the viewpoint of those who wrote it.
The research conducted by Polish and German historians can not change the facts of what happened regardless of their own personal feelings towards the matter. They are simply doing their jobs by looking for the truth and presenting it as they find it. Everyone is allowed to have their own opinions about the results.