The % means that the tank will spawn at the same position of the map, not the same amount of times. I'm not overly sure how your game played out - if you actually finished or not - but depending on how well the other team is going it's to be expected.
While I know most people may have noticed the update by now!!!! I'm still going to post the patch notes for those who want to know and are at work (suckers).
January 14, 2009 - Left 4 Dead Update Released
Left 4 Dead has been updated with a host of fixes, tweaks and new features, including...
I'm worried to say it here... but if you join the spectators then back to the infected the infected you were just playing becomes an AI controlled special. With some games I've been in some tools have spawned easily 20 specials on top of us.
Bloody losers ;(