When you use the Use Column Spacer Image feature with layout tables, FrontPage does something special to the code: It adds an extra row to the bottom of the table, and it inserts a GIF called MsSpacer.gif to the columns you want to keep propped open.
A residual effect is that when you look at the table height in the Layout Tables and Cells task pane, you'll see a very small contradiction for the height.
Table height, 300 pixels, plus 1-pixel-high row = 301.
This means the table was originally specified to be 300 pixels high, but FrontPage is displaying it at 301 because the extra row of spacer GIFs is one pixel high.
The column-spacer row at the bottom of the layout table.
This height difference is so small that you will hardly notice it by looking at it. But it's good to be aware of this contradiction, nonetheless.