Thats what i thought. This kinda "trying to be funny" rule had to mess my brain all up:
"What makes a tunnel special is that a player is never quite certain just how long the route he is trying to claim will be!"
This rule just simply states that it is possible to pay more cards for the tunnel, than it is worth.
But speaking about "trying to be funny" rules that confuses the reader totally, you should check Tichu rules. I had to read them three or four times before starting the first play...