On a saxophone you might play one note, let’s say a ‘C’ and be in tune.  The needle on the tuner perfectly in the middle!

Then you could play a C an octave above it and be sharp.

And you could play a G# below and be flat.

The saxophone may have a slightly different tonal center for any note you play.

It’s up to you to play in tone.  As you play longer, you will become more familiar with your saxophone and learn what to do to play in tune.

The saxophone’s construction is an optimized design so that all notes can be basically be played in tune (if you also have good embouchure and control).