Ten is the only one that can be written in figures as well as letters. On the other hand, Placebo is the only one in a foreign (Latin) language. Take your pick.
No, I was wrong. It is based on 'X'. Ten is X in Latin. Kisses can be written XXX. When you vote you place an X in the box. Incorrect is marked with an X rather than a tick. I've got a hunch that doctors use XXX to indicate a placebo. On that basis, Unknown would be the odd one.