• All green boxes below are exactly the same size (style declares width and height).
• Boxes are defined as absolutely positioned and sized DIV elements.
• Box "two" contains an inner DIV with a 4px border one all sides and height of 100%.
• Box "three" contains an inner DIV with a 4px border on bottom and right sides and height of 100%.

The problem is that the inner DIV is too tall - by the sum of the thickness of the top and bottom borders.