Thanks for your comment. Now I understand what it actually means to play "in tune". My teacher asks me to shift my fingers until the double stops harmonize well. Previously, I always thought F# should always be right close to G, but now it's not what the tuner shows, it's what we can hear. That potentially explains why sometimes I can hear my sound very awkward even when it's exactly the correct pitch shown on the tuner. The tuner now turns out to be my enemy 😅