All materials flex under load just like wood or rubber. Even steel flexes a little although it may not be apparent to the eye. This is why rigidity of setup is important and why spring back happens. Keep the concept of flexure in your mind at all times and you will be a better machinist. R.S.
When feeding a boring bar into a piece of material, the tool has a tendency to lightly bend towards the center of the hole as if it was made of some very flexible material. This explains why when you re-feed a boring bar into the hole at the same exact location - without changing the offset - you will still remove some more material.
This phenomenon is called spring back and you must be aware of it when you are using this tool on a milling machine. Also be aware that the longer and smaller diameter of the shank of the boring bar the more flexure or bend. Thus it may be required to take multiple spring passes to remove all flexure or bend in a deep hole.