I think GP means the percentage of homeless people that have a mental illness is similar to the US, not the percentage homeless compared to the total population.
I got 30800, and I am a native Dutch speaker. Though I have to say that I consider English my co-native language. Most of the media I consume is in English, and in my work environment (IT) English is the default language.
But most of the words in this test I knew from reading books, as it is one of the few places where using a very wide vocabulary is not frowned upon.