You can cheat when adding or subtracting votes. Please try out this sequence for one of the answers:
- Vote up
- Vote down
- Undo
- Vote down
If you now refresh the page (browser) you will see that one vote has been deducted. Now repeat the above steps for the same answer. You will see that another vote is deducted. This way you can subtract the next votes. You can do the same for adding voices.