TD height in the presence of rowspan

I'd like to understand the specific logic in why
the following effect is happening in IE6 through
IE8, while displaying as expected in FF. It is
easy to work around (by replacing the 3 rows
of the right column with a table) but I am
after understanding here.

The following 4 cell table has a left column with
a fixed height of 300px and I thought to squash
the upper two rows of the right column by
putting style='height:1%' in the relevant TDs.
This worked as expected in FF, giving me a
bottom right cell nearly as tall as the main
cell, but in IE the right column cell heights
are evenly distributed.

Any explanations for what is happening under IE?
Csaba Gabor from Vienna
Here's the table:

<table border><tr>
<td valign=top style='height:300px'
rowspan=3>Main cell</td>
<td style='height:1%'>Cell 0,1</td>
</tr><tr>
<td style='height:1%'>
Cell 1,1</td></tr>
<tr><td>Cell 2,1</td>
</tr></table>