The skeleton of a binary object is a centerline (see here).
This means that skeleton of a binary object should (must) have exactly the same thickness everywhere, otherwise its not a line - lines have no “thickness”. This is why the mean and max are the same: because the thickness is the same everywhere.
Because of this, I don’t think it’s meaningful to measure the thickness of a skeleton.
That being said, I’m surprised you’re getting a value of 2. (I might expect 1, presuming the skeleton is measured on a pixel grid with a one-pixel “thick” mask. I’m not familiar with the BoneJ implementation, but it may be that your choice of connectivity rule could affect the value of thickness you get.
defines the thickness at a point as the diameter of the greatest sphere that fits within the structure and which contains the point.
Hope this helps, but post back if you have more questions,