Mystery gap between <DIV> blocks

I recently tried for the first time using blocks to display scrolling tables. I used one DIV block for the headings and another for the scrolling area. On one of my pages it worked fine, but the other place I used it, there is a gap between the heading and data sections. The gap only appears if there is over a certain number of table rows, and the more rows there are, the bigger the gap. A simplified page containing the offending code is at http://members.dslextreme.com/users/rreese/code/div-problem.htm

Any help on this is greatly appreciated !