Upon loading a saved FBT calibration, it won't be perfect and the hip tracker will often be noticeably offset (since that's the most obvious one).
To reproduce:
  1. Calibrate FBT, making sure your spine is straight
  2. Change world
  3. Check if spine is still straight
Might also work when changing avatar...
More likely than not, the spine won't be straight anymore (the hip tracking will be offset a couple cm to a side for example). Probably does this with every tracker as well.
I think not having the exact same pose when leaving the world and loading in might be a factor, but I didn't test that theory.