This reminds me of my experience with people in China when I was there. My friends there told me that many young people in China do understand English. But every time I approached them with "Do you speak English?" They usually took two steps backward and replied "no". They hesitate speaking English (because they find it difficult) despite knowing enough to serve my purpose.
I have had similar experience with "Do you know maths?" in the US.
I have had similar experience with "Do you know maths?" in the US.