too many collisions in random number generator collision test

I'm expecting lKolizji variable to be around 128, but it's much higher for large ammount of generated numbers and "boxes". The results for smaller numbers are good. I have no idea why is this happening. Here is my code with example parameters that give wrong answer. Example of good result(around 128) is int lPrzedzialow=1000000; int iLiczb = 16000;

#include <iostream>
#include <gsl/gsl_rng.h>
#include <stdlib.h>
#include<cmath>
#include <algorithm>
using namespace std;
int main (void)
{
//Random number
   unsigned int seed=2596524;
   gsl_rng * r=gsl_rng_alloc (gsl_rng_mt19937);
   gsl_rng_set(r,seed);
   gsl_rng_env_setup();
//Parameters
   int lPrzedzialow=10000000000;//number of boxes
   int iLiczb = 1600000;//number of random numbers
   int z,lKolizji=0;//lKolizji holds collision number
   vector<int> lwKomorkach(iLiczb);//number of boxes of random numbers
   long double dlPrzedzialu=1./(lPrzedzialow);
//number of box of a random number 
   for (int i = 0; i < iLiczb; i++)
   {
       lwKomorkach[i] = floor((gsl_rng_uniform (r)/dlPrzedzialu));
   }
//sorting
   sort( lwKomorkach.begin(), lwKomorkach.end() );
//how many collisions
   for(z=0;z<=iLiczb-1;z++)
   {
       if(lwKomorkach[z+1]==lwKomorkach[z]){lKolizji++;}
   }
  double pdf[lKolizji];
  pdf[0]=exp(-128);
  double spdf=exp(-128);
  for(int h=1;h<lKolizji;h++){
     pdf[h]=pdf[h-1]*128./(h);
     spdf+=pdf[h];
  }
  double pwyzsze=1.-spdf;
  cout<<endl<<lKolizji<<" "<<spdf<<"  "<<pwyzsze<<endl;
  gsl_rng_free (r);
  return 0;
  }