Birthday Paradox

i was assigned a short project in c++ today
and i thought that i would share the code that calculates the birthday paradox

#include <iostream>
#include <cstdlib>
using namespace std;

double birthdayCollecter(int B[],int n,double t)
{
    int C;
    C=0;
    for (int h=1; h<=t; h++)
    {
        for (int i=0; i<n; i++)
        {
            bool breaker = false;
            B[i] = (rand() %365 + 1);
            cout<<"\nB["<<i<<"] = "<<B[i];
            for (int j =0; j<i; j++)
            {
                if (B[j] == B[i])
                {
                    C++;
                    cout<<"      "<<C;
                    breaker = true;
                    break;
                }
            }
            if (breaker == true){break;}
        }
    }
    return C;
}
int collectPeople()
{
    cout<<"how many people would you like to check at once : ";
    int people;
    cin>>people;
    while (people>100)
        {
        cout<<"\nSorry pick a number lower then 100, the percent it too high otherwise\n";
        cout<<"how many people would you like to check at once : ";
        cin>>people;
        if (people<100){return people;}
        }
    return people;
}

int main()
{
//this is the intoduction message, it asks the user to input the wanted values
    cout<<"\nwelcome to the birthday paradox calculator \n";
// the number of people to be tested is input and checked   
    int numPeople = collectPeople();
    cout<<"\nok you want to check "<< numPeople <<" People \n";
// the number of wanted simulations is imput   
    cout<<"how many times do you want to run the simulation : ";
    double trials;
    cin>> trials;
    double Count;
    int birthdayArray[numPeople];
// The count of how manny people share birthdays is defined by the Birtday Collecter function
    Count = birthdayCollecter(birthdayArray,numPeople,trials);
    cout<<"\nthe Probablility of two people in a room of "<<numPeople<<" sharing a birthday is : "<< (Count/trials)*100<<"%" ;
    cout<<"\n\n";
   

    return 0;
}

Why is it called the birthday paradox? I have trouble identifying the paradoxical part.