"For a grid of cells, first, the vertical and horizontal BoundSprings are computed separately. The vertical BoundSpring is determined by stacking vertically the BoundSprings of each of the rows, and the horizontal BoundSpring is determined by stacking horizontally the BoundSprings of each of the columns. (Of course, the BoundSpring for each row or column is determined as described in the paragraph above.) The vertical and horizontal BoundSpings are finally combined to compute the BoundSpring of the grid."
Well, it's a feature of the (non-coreJava) library you're looking at. And, as it happens, the page you linked to contains an explanation. See the section starting "A BoundSpring encapsulates the geometrical properties of a cell in terms of its minimum, maximum and preferred width and height."