Why do 'I'm up for it' and 'I'm down for it' mean the same thing

When to Say 'I Love You' Varies: Why, How to Tell, More

Why do people say 'I'm down' when they are up for doing something? - Quora

How did the English language allow “I'm up for that” and “I'm down for that” to mean the exact same thing? Where is the origin of these phrases? - Quora

Meaning of I'M DOWN FOR IT and I'M UP FOR IT - A Short English Lesson with Subtitles

What is the reason behind people using 'I'm down' instead of saying they agree to something? How does the meaning of 'I'm down' differ from simply saying 'I agree'? - Quora

What is the difference between 'go down to' and 'get down to' (some location)? - Quora

What's the difference between being 'up' or 'down' for something? - Quora

In English language slang, what does it mean if someone is 'down for the count'? - Quora

Kim Kardashian: If I'm Doing It, It's Attainable — Interview

Why do 'I'm up for it' and 'I'm down for it' mean the same thing? - Quora

How did the phrase 'I'm down' originate? - Quora