I think I can explain the German stability map
German colonies:
green and yellow territories in South America, Asia (East Timor, some area in China) and Africa all represent former German colonies - most of them were very short-lived though as Germany gained most of its colonies very late in the 19th century and lost all of them in 1918. Please note that even till today a good amount of the upper class of Namibia is of German origin.
German emigrants:
Green and yellow territories in North America and Eastern Europe represent German people who emigrated into these regions - most notably the large immigration in Northern US and Canada during the 19th century but also all the German founded cities&villages in today Romania, Hungary, Russia, Ukraine, Poland,Czech, Baltic States you name it.
Today these former German pockets either peacefully absorbed into the local culture or the German population got forcefully evicted after 1945 and then these regions were resettled with Non-germans. Nevertheless for centuries especially these Eastern European regions were inhabited by Germans, who even lived and spoke only like Germans (even developing their own version of the language) and often created a parallel state within the state.
The dark green territories in Eastern Europe represent the inhabited regions which were at times even part of first the Holy Roman Empire and later the German nation state. Most of these got lost after 1945 and the population got removed, simply because the German population had and was endangering the stability of the newly founded Slavic states as the Germans never truly accepted to be ruled by Non-germans. Especially the current East-Poland got often invaded by Germans, while West Poland (Silesia, Pomerania) , Czech Republic (Bohemia) and North Poland/Baltic(Eastern Prussia) were an integral part of Germany/Austria most of its history in the last millenia.
Clash with other cultures:
Red areas in today Germany are clashes with other cultures. While German expansion often went unchecked in Eastern Europe it met strong resistance in Western Europe. France, Germany and later Netherlands (a culture very much related to German culture) constantly fought over Western Germany/Eastern France/Lower Countries for centuries until the current borders were established only in 1945. Therefore possession of these territories was highly unstable and foreign claims were common.
Germanic origin:
Yellow territories in Scandinavia are due to the common roots of Germanic and Scandinavian civilization. The original Germanic tribes were very much related to the Scandinavians in language and ethnicity and even though the split occurred 2000 years ago there are still similarities in values, morale, religion, personal features etc.
Hence it could be assumed that Scandinavians would be more comfortable (long term) being part of a German empire then lets say of a French or Chinese Empire.
History has shown that the resistance to German rule during WW2 was much bigger in Russia and France than in Scandinavia - I suppose this could justify the yellow territories in Scandinavia - please note that Norway/Denmark as the Viking heartland is red for the Germans and in fact the Germans faced some substantial civil resistance in Denmark during WW2
Holy Roman Empire:
Yellow territories in Italy are there because of the history of the Holy Roman Empire. For centuries large parts of Italy were formally ruled by a German Emperor and only when the empire slowly disintegrated the Italian states regained independence. However even during the height of the empire the German rule was often more formally than directly noticeable, as in general the execution of power was less absolute during the Middle Ages. Still the rich Italian city taxes did pay taxes to the Emperor without much rebellion and at the end this is was matters isn't it
So by and large the German stability map is a very neat conclusion of German history, very well thought of and rich in detail.