Poisson
Poisson
[Cueball is talking to Black Hat. Cueball has his mouth wide open and has both of his arms up.] Cueball: I'm a poisson distribution!
[Same scene, except Cueball has only one arm up.] Cueball: Still a Poisson distribution! Black Hat: what the hell, man. Why do you keep saying that?
[Cueball's face is gone, and he is not holding any arms up.] Cueball: Because I'm totally a poisson distribution. Black Hat: I'm less than zero.
[Cueball is gone. Black Hat is now whistling.]

Cueball expresses himself as a Poisson distribution .
A Poisson distribution is a distribution that shows the probability of a given number of events occurring in a fixed interval of time or space. The X axis typically represents the "number of events" while the Y axis is a decimal representing the probability (i.e. 0.5 for 50% probability) a given number of events will occur in that fixed interval of time or space. It is commonly represented by a bar graph or a scatter graph (sometimes with a line connection to show a trend, even though there is no actual value for non-integers).
What's important to note for this comic is that this distribution only has data points on non-negative integers and is not continuous through decimal numbers or (as the image text tells us) negative numbers because events can't occur 0.3 of a time, or −2 times.
After implying that the concept of a person being a mathematical distribution is irrational, Black Hat suggests he is "less than zero". Since the Poisson distribution doesn't exist or has no value at negative values, Cueball either leaves or disappears magically.
Hence, the punchline is the same as the title text: Cueball doesn't exist to Black Hat anymore, because he has a value less than zero. Another one of the early comics where Randall explains the joke in the title text.
Also, because a Poisson point process is memoryless, the figure claiming to be the distribution may simply be repeating the fact as a reference to this.
[Cueball is talking to Black Hat. Cueball has his mouth wide open and has both of his arms up.] Cueball: I'm a poisson distribution!
[Same scene, except Cueball has only one arm up.] Cueball: Still a Poisson distribution! Black Hat: what the hell, man. Why do you keep saying that?
[Cueball's face is gone, and he is not holding any arms up.] Cueball: Because I'm totally a poisson distribution. Black Hat: I'm less than zero.
[Cueball is gone. Black Hat is now whistling.]
