Problem with viewportOffset

[jammypodger]

I have a problem with viewportOffset.top reported on a td on a table
inside a scrolling div. I have tested in IE 6 and 7 and both fail,
while FF 2 is ok.
When the table is displayed and has not been scrolled the
viewportOffset reports correctly for td elements, as soon as it has
been scrolled IE reports incorrectly, while Firefox reports correctly.
An example of this can be seen here ;-

http://193.111.226.249/example/example.html

Roll your mouse over the top row of the table to see the offset, then
pull the scroll bar to the bottom, and roll your mouse over the top
row again. In IE the viewportOffset.top is now negative.

Am I using this incorrectly, or is this behaviour to be expected?
TIA
jammie