100%?

I am baffled by the idea that there can be a 100% chance of anything. "Taxes," suggested a friend this morning. Okay, fine: death and taxes. But anything else? Even the fact of the sun rising, the earth continuing to spin, the sky being up and the ground being down, gravity, entropy, cereal ...

Surely the scientists at weather.com are rounding up? I would be much more comfortable with a percentage that allowed for the element of sheer randomness and chaos in our lives -- 99.8% even. Am I alone in this?