When you draw a box around a group of cells, it is based on the positioning of the grid between the cells and their neighbours.
If you are creating a complex document, where the width of columns varies as tables and text appear in the document, you may encounter some problems with the drawing of boxes.
The simple solution to this is to keep an extra row in the same column format as the table above and below the table, as shown in the screen below. These act as a buffer between the table and the rest of the document and ensure that its box is always displayed and printed correctly.
You should not encounter this problem where tables are added to documents where the surrounding column is the full width of the page; it only occurs when tables are embedded in worksheet type documents with variable column widths.