You might be right that some of the Germans or others living here could also have US dual citizenship, but I guess there are not all too many of them. Still, the number provided by the US agency seems to be overblown, because the Austrian registry is very accurate and up-to-date and STATISTICS Austria conducts a mini-census each year with strict quality controls to get accurate registry figures.
As for the increase in Germans, there are mostly 3 reasons for that: The first is that many students from Germany moved to Austria in the past 10 years, because Germany has the numerus clausus, whereas Austria does not. It means that in Germany, universities only accept students with best grades, whereas Austria has free university access for every high school student that passed the final exams. We call the German students who come here to study "numerus clausus refugees" 
Another reason are retirees who prefer the Austrian landscape to the grey Ruhrpott. A third reason is labour: Many east-Germans come to Austria for work (but in recent years they stay increasingly in Germany, because the labour market is now much better in Germany than in Austria).
The strong immigration from Eastern Europe pretty much explains itself with the EU East Expansion from 2004-now.
The uptick from former Soviet countries are mostly Chechens, of which many came to Austria after the Chechen Wars, also many Georgians and Ukrainians recently.
I have no clue why Mongolians increased that much ...
Osttimor = East Timor
And in "DR Congo, ehem Zaire" - the word ehem. is an abbreviation for "ehemalig" (which means "former").
The FVAP did not produce the estimates. They commissioned a study on how to estimate the number of overseas citizens. As part of the study, they produced estimates for individual countries. In the paper they noted the State Department estimates, but said that the State Department did not disclose their methodology, or produce country-level estimates. Yet in your initial message you accepted the State Department as gospel.
In calibrating their model, Austria did get credit for using a registry, but it appears that they lost credit for not counting dual citizens.
It is very interesting to compare the statistics for place of birth vs. citizenship. The number of USA-born has grown at a much faster rate than than the number of USA citizens. The difference is particularly noticeable based on age (here I am using Canada+USA, since Austria does not show these statistics for the USA alone).
Only 40% of the North American-born 6-14 YO are recorded as citizens, yet as a matter of law almost all USA-born persons are USA citizens (the exceptions are those born in embassies). Even among 15-24 YO only 60% of the North American-born are recorded as US citizens.
Only until you get to 65+ is the number of NA citizens is greater than the number of NA-born (108%). So among those born before, during, or immediately after WWII you have persons who were naturalized and then moved to Austria (either returning or having born in other countries).
The ratio of USA citizens to USA born is highest in Vienna and Salzburg, and lowest in Burgenland and Vorarlberg. This suggests that the registry is best at recording students and 20+ working who are both USA-born and USA citizens, and less good at returning Austrians, whose children are USA citizens (though they may not be practicing Americans).
The increase in citizens from the former Soviet Union is pretty general:
Baltics:
Estonia 9X, Latvia 8X, Lithuania 7X
Eurasia:
Russia 9X, Belarus 7.5X, Ukraine 5.5X, Moldova 5.5X
Transcaucasia:
Azerbaijan 10X, Georgia 9.5X, Armenia 6.5X
Central Asia:
Kazakhstan 14.5, Turkmenistan 12X, Kyrgyzstan 11X, Tajikistan 9X, Uzbekistan 6X
I had already translated Timor Leste. Most of the names are transliterations. So I was trying to understand OH-sti-mer, and not trying to translate Ost.
In English, "ahem", is used as a way of offering disagreement or a correction.
So if someone said "Zaire", a mild corrective response would be "ahem, DR Congo". That is it was somewhat of a loan word, but seems to be more in the way of throat clearing or stylized cough.
