Interesting that countries where English is an official language scores lower than countries where it isn't, like Singapore, Malaysia, and South Africa.
The full report had a list of cities though, where Kuala Lumpur scored higher than Malaysia, so that's an interesting phenomenon how it's a city thing rather than a country thing.
As others have mentioned, English is an official language in Singapore and South Africa.
As for Malaysia, English is not an "official language" however it is an official language in one state, and it is still an important language in government - particularly in the judiciary, where many cases are still conducted and judgements written in English. More importantly, it's the primary language in most larger private-sector businesses particularly in cities like KL where Malay is less dominant.
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edit: unless we're interpreting you wrong, and you mean that you're surprised that Northern Europeans do better than some Commonwealth countries - ignoring the obvious massive bias issues with this "study" as a whole, I'd remember that Europe has higher quality education in general than the poorer Commonwealth countries.
Yeah, sorry, I should have flipped the is and isn't so the examples followed the right one...
And you're probably right that the bias in the study skews the results.
Another possible angle is how you measure "correct" English. American English and British English are technically two separate languages with separate rules. Should you measure the correctness of a Singaporean against either of those? Or should you measure them against some Singaporean English standard instead?
FWIW, the Singaporean government actively tries to encourage "Good English" rather than Singlish, much to the dismay of many.
The Malaysian government doesn't do quite the same type of attempts at social engineering as the Singaporeans, so you don't see quite as many official denunciations of Manglish.
Even though South Africa has 11 official languages, English is the most official language in South Africa. When speeches are delivered by public officials, they are 99% in English. All official documents have English in it. Even the government official website is in English [1]. It is the only compulsory language at school. All private and urban schools teach English as a first language, then other languages as additional languages. Parents this side make sure their kids can speak English before they speak their own indigenous languages. Not mine though.
Singapore also has a lot of (temporary and permanent) immigration, and those immigrants may need to take ESL courses to adapt to Singapore, that would reflect on these results.
The full report had a list of cities though, where Kuala Lumpur scored higher than Malaysia, so that's an interesting phenomenon how it's a city thing rather than a country thing.