because the postbit is also used for the private messages.
I had this problem with a "pm user" button in my postbit, showing up in pms
fixed it by adding
<if condition THIS_SCRIPT == "showthread">
your code here
</if>
so if a user is viewing the postbit from showthread.php, it will show the ad.... and if the user ISNT viewing the postbit from showthread.php, it will NOT show the ad.
hope it helps.