Why do fat chance and slim chance mean the same thing?

I dunno. Maybe because "slim chance" is literal and "fat chance" is sarcastic.

they dont mean the same...
slim chance means a small possibility
where as fat chance means there is no possibility

They made it teh same thing were あなた would ask.... lol no just playin..
I think there the same because it is a contrast. Like Cool And Cold. Huh, Hope I helped..

people say fat chance usually in a sarcastic way,cause its not true and slim chance is what it is,very small:)

just because didnt あなた know they made a law that 発言しました that

Because fat chance is sarcastic.
eg:Boy: Mum, can I go outside and play in the rain? (It's a sunny day).
Mum: Fat chance, kid. または Slim chance, kid.