European Surnames - Map Quiz

What's the source of this data? Several countries seem to be wrong or have typos (for example Slovakia's most common surname is Horváth, not Varga; Hungary's is Nagy, not Napy and there are more like these two)

Also for me Belarus seems to be bugged, I guessed Ivanov but it marked it as incorrect as below:

"This surname is:

Ivanov

you guessed Ivanov"

Too bad, the concept was nice. When you correct the errors this will be a great quiz!