Windows XP wireless network list behaviour

I built a cantenna and it seems to work great but with it I noticed an odd
behaviour with the list of networks Windows sees:
If I point it in a direction and search for wireless networks, get the list,
then set the antenna face down on a sheet of foil to block all signals and
search agaon, it keeps all the networks it found before! Now if I keep the
antenna on the foil, then disable then enable wireless and search, it will
find nothing.

This is annoying because if I point the antenna in another direction, I have
to disable and enable then search to find acess points unique to that
direction.

Why does Windows not dump the previously found network list when searching?

PS: I'm running service pack 2 XP Pro.