Because it lists countries with native English speakers and the number of same for the language in question on that page.
You should generally be able to get the relevant information from the country’s own statistical service. For example when looking at Finland we find that in 2005 there were 5 255 580 people living permanently in the country, of which 144 344 spoke foreign languages, ie. something other than Finnish, Swedish and Sami. By using Statistical center’s database we find that 8 928 of those had English as their language, making it the 5th largest language after Finnish, Swedish, Russian and Estonian. So only roughly 0,17% of people living in the country spoke it, though since 1990 the number of its speakers had more than doubled. Note that of the whole population, some 113 852 had nationality other than Finland’s, which makes it probable that most of those who had English as their language were also foreign nationals.
It wasn’t the OP’s question, but since the ability to speak English as second (or third etc.) language by different European nationals was brought up, it might be interesting to look at a few statistics about that. European Commission’s Eurobarometer does public opinion analysis, usually by the large standard surveys but they’ve also done numerous special surveys , occassionally asking people to rate their perceived language skills in addition to explaining their attitudes towards foreign languages. Language-questionnaire Eurobarometers include (PDF files follow) Ref. 147 in 2001 (summary , original), short, but informational survey Ref. 237 in 2005 and the massive attitude-focused Ref. 243 in 2006 (summary and the whole thing (big PDF)).
The results support the common wisdom that most Northern Europeans can be expected to know some English, while most Southern and Eastern Europeans don’t know the language at all. Also, the British probably are the worst in EU when it comes to languages, less than third can conversate in language other than their own. The best English speakers in continental EU are the Dutch, Swedes and Danes, where 80% to 90% of population knows English, and usually at least in ‘good’ level. It’s also correct that most Dutch, about two thirds, speak German, though only about one in four speaks French. Similarly, the majority of Danes seem to speak German, too. In the knowledge of English, the Germans, Austrians, Belgians and Finnish are next, but here only a slight majority knows it, and even those who do, often not as well as the Dutch English speakers, for example. Going south, one third of the French know English, while in Mediterranean countries the number is generally lower than that, especially in Spain. In Eastern Europe the knowledge of English is equally rare. In the South and East it’s also more typical for those who speak English only know a basic level of it. While this kind of opinion polls cannot give a very accurate picture, as can be seen from how some of the numbers vary considerably from survey to another, and some people no doubt overestimate their skills while others underestimate them, the results as a whole are a rather good estimate of what kind of language proficiency you can expect in different European countries.
My experience with Spanish people is that many of them underrate their knowledge of foreign languages. We’re definitely not the best English-speakers in the world but many of my coworkers have been surprised by how well they fared in international meetings. They may speak “spaguetti English” but it’s enough to understand others and to make themselves understood.
My own Lilbro didn’t believe me when I told him he was slightly above “3rd grade EOI level” (roughly equivalent to a Cambridge’s First Certificate, level-wise, and sufficient for most business and pleasure purposes other than teaching and translation) until he came to visit me in Philly and saw that he understood well over 90% of the conversations he heard, TV, etc.
Speaking of English in Finland… I went to a Värttinä concert tonight… the three women from Finland all spoke to the audience in English, reasonably fluent. And they had the sexiest sounding accents I’ve ever heard!