This sounds semsible - although I also know some German, Scandinavian or Dutch descents also use the general "American" label in the census and so the figure could be a little lower?
It looks quite astonishing when comparing the set of data between New Zealand and the United States: overwhelming of New Zealand Europeans have majority British/Irish ancestry, while it is not so for American whites.
New Zealand and Australia were part of the British Empire and are still members of the British Commonwealth, as is Canada, while America became independent in 1776. As a result, there was greater attraction for British settlers to nations under the Union Jack and less of one for non-British Europeans. Additionally, both the U.S. and Canada are closer to Europe than are Australia and New Zealand. With respect to Dutch and German preference for America as opposed to the British Empire/Commonwealth nations, you must remember that there was a strong Dutch and German presence in colonial America, concentrated in the Middle Atlantic colonies of New York, New Jersey, and Pennsylvania. Even today, the concentration of German Americans in Pennsylvania is one of the highest in the nation.
It varies a good deal by region. Lots of Germans in Texas, for example. Quite a few Norwegians in the Northwest. Lots of Johnsons everywhere!