std::unordered_map insertion times vary widely

When looking at the average insertion times in an std::unordered_map, I saw that the insertion times varied very widely. Here is my code:

int main(){
    unordered_map <int, int> m;

    for(int i = 0; i < 200000; ++i){
         double start = clock();

         m[i] = i;

         double time = (clock()-start)/(CLOCKS_PER_SEC/1000);
         if(time >= 1){
              cout << i << endl;
         }        
    }

    return 0;
 }

Every time I run this code, I receive the exact same output:

12982
26266
53200
107896

Every other iteration runs in less than .01 milliseconds.

Now, all the books/websites I've read have said that inserting to a std::unordered_map runs in O(1) time, so why do the times vary so widely here? Is it something to do with the hash function or the size of the map?