What's the difference between a 'chance' of a thunderstorm and a 'risk' of one?

I have heard meteorologists say: 'A chance of a thunderstorm on Tuesday with sunny periods following. On Wednesday, there is a risk of a thunderstorm developing in the afternoon. (Or vice-versa). I know that there is a difference between them in, let's say, poker, but what does it mean when it comes to the skies. They seem to be interchangeable. Why is that?