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Compare to this map of Italian ancestry and you'll see high percentage of German ancestry all over the USA.
|Rank||Ancestry||Number||Percent of total|
This one is even more revealing:
Plurality ancestry in each state, ranging from 11.8% (FL) to 43.9% (ND).German American Mexican Irish African Italian English
This means, I believe, that the state is colored to indicate the largest national group. For most of the country it is German.