The joystick is an entirely analogue component and, due to the way it is held central by springs, can sometimes 'creep', very slightly, away from the center point and this results in small pitchbend amounts being transmitted.
In Settings>UI & Controls, navigate to Joy Centre calibrate. There are also X and Y calibration options in the same menu.
You can also do this via the MODALapp.
Please take a look at this Modal Minute that gives you some more information.