Why does "for that" change the meaning if combined with "up," but not with "down"?

(1) I’m up for that = someone stating their own interest/availability for what “that” refers to
Bob: “Hey, wanna go get coffee?”
Zack: “Yeah man, I’m up for that.”

(2) I’m up = a) a person physically stood up or, b) a person is now conscious
Bob: “Hey, wanna go get coffee?”
Zack: “Yeah man, I’m up.” *atypical

Bob: “Hey, you gonna get outta bed sometime today?”
Zack: “Yeah man, I’m up, I’m up.” <- typical to my dialect

Grammatically, why does adding the prepositional phrase “for that” change the semantic meaning between (1) and (2)? Is the change because of the words “for” and “that” and their denotations? Or is it because adding a prepositional phrase (“for that” in this case) to the phrasal verb (“to be up”) is triggering the semantic shift?

As a point of contrast, the meaning does not significantly change between:

(3) I’m down for that = someone stating their own interest/availability (ie for the “that”)
Bob: “Hey, wanna go get coffee?”
Zack: “Yeah man, I’m down for that.”

(4) I’m down = someone stating their own interest/availability
Bob: “Hey, wanna go get coffee?”
Zack: “Yeah man, I’m down.”

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Some additional background info, from another question I asked, in the comments, @WS2 added a valuable distinction:

@wanderling To me I’m down for that = My name is on a list/it has all been arranged that I am coming. However, I’m up for that = I am keen to be involved. Where do I sign?

And there is a question on the etymology for “I’m down for that” here at What’s the origin of “I’m down with it”?, which could be similar enough to “I’m up for that” to explain its etymology as well.

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