Why are my 'randomly' generated numbers are increasing every time I run the program?

I'm trying to get randomly generated numbers by using srand() and rand() but with no avale: every time I run the program it increases by a certain amount of numbers. But if I use a for statement, no pattern can be seen.

For example, if I use the code provided below and run and close the program 10 times, the output will be:

6
32
64
91
13
62
...

Note: one strange thing I noticed about this that when I unfocus or minimize Visual Studio and close the Command Prompt, the next time I run the program the number increases by more than 20, but, if I don't unfocus or minimze Visual Studio the number increases by just little over 1-5. Why's that?

#include "stdafx.h"
#include <iostream>
#include <cstdlib>
#include <ctime>

using namespace std;

int main()
{
    srand (time(NULL));

    int random_number = rand () % 100 + 1;
    cout << "Random number: " << random_number << endl;

    return 0;
}

But, if I use this code:

#include "stdafx.h"
#include <iostream>
#include <cstdlib>
#include <ctime>

using namespace std;

int main()
{
    srand (time(NULL));

    for (int i = 0; i < 10; i++) {
        int random_number = rand () % 100 + 1;
        cout << "Random number: " << random_number << endl;
    }

    return  0;
}

the numbers don't have a clear pattern and I get the output:

31
10
81
66
74
14
6
97
39
23 

They increase and decrease randomly by random amounts. What seems to be the problem here?