Illogical English #7

How can a slim chance and a fat chance be the same?
Both a “slim chance” and a “fat chance” mean to have little possibility. For example, “I think England have a fat chance of winning the next World Cup”.
The difference is that a “slim chance” is traditional, meaning a narrow band of possibility, and much later a “fat chance” was added as sarcasm (which often uses opposite meanings) to stress the situation more. So, in my sentence above, it means I believe there is absolutely no chance, but if I had used “slim”, then it would be at least very slightly possible.