Mathematical joke regarding three people leaving a house I don't get this mathematical joke - can someone explain?

From Wikipedia:
A physicist, a biologist and a mathematician are sitting in a street café watching people entering and leaving the house on the other side of the street. First they see two people entering the house. Time passes. After a while they notice three people leaving the house. The physicist says, "The measurement wasn't accurate." The biologist says, "They must have reproduced." The mathematician says, "If one more person enters the house then it will be empty.

 A: input = 2 
output = 3 
so, the system contains 2-3= -1 people.
So he says if another person enters the house would have -1 + 1 = 0 people :) 
A: Note that there are $2$ people who enter the house at first. Then $3$ people leave the house.
The joke probably lies in the fact that the mathematician assumes that there were initially no people inside the house (The others do too), and so then there are:
$$2-3=-1\text{ people}$$
In the house at the moment. Therefore, if one person enters the house, the amount of people in the house is:
$$-1+1=0 \text{ people}$$
Thus, the house will be empty.
A: I think the point of the punchline is that the mathematician simply solves the math problem as observed, and appears totally unconcerned with the impossibility of having -1 people in a house in the real world.  
Sciences like physics and biology are about explaining the real world, but in mathematics explaining the real world is not a requirement.  
A: From my understanding, it's just a take on stereotypes of physicists, biologists, and mathematicians.  Physicists are generally concerned with real-world measurements, and may often attribute issues to experimental error (counting the wrong number of people here).  Biologists end up attributing the extra person due to sex (reproduction).  The mathematician sees $2-3=-1$ people in the house, so if one more person enters, there will be $-1+1=0$ people.
