Hi Zero_1
I think you are checking global co ordinate between end point to end point of bellow at operating condition.In that case you will find the lateral difference due to angular rotation of hinge point from simple geometry as Mr Dave already mention.If you use 2 or more bellow in different direction you will see more significant difference.
Obviously it is valid for finite element bellow only. If model with zero length bellow it will not happen.
So your understanding mention above is correct in my point of view.