Are i < N+1 and i <= N different?

Question

I used to know that There is no difference between i <= N and i < N+1
However, when I enter 6 6 to program.

if i <= N then it print

1 6 6
6 1 1
2 3 3
3 2 2

otherwise

1 6 6
6 1 1
2 3 3
3 2 2
3 2 2
2 3 3

I can't figure out why it make a difference

#include <iostream>
#include <cmath>

using namespace std;

typedef long long LNT;

LNT gcd(LNT a, LNT b)
{
    if( b == 0)
        return a;

    return gcd(b, a%b);
}

int main()
{
    LNT red, green;
    LNT GCD;
    cin >> red >> green;

    GCD = gcd(red, green);

    //for(LNT i = 1; i<sqrtl(GCD)+1; i++)
    for(LNT i = 1; i<=sqrtl(GCD); i++)   // <- This Line cause the difference 
    {
        if( GCD % i == 0)
        {
            cout << i << " " << red/i << " " << green/i <<endl;
            if( i != GCD/i )
            {
                LNT k = GCD/i;
                cout << k << " " << red/k << " " << green/k <<endl;
            }
        }
    }
}

Answer 1

This is true only for integer values. As sqrtl returns long double, in case it's fractional then for the fraction it will still differ if you compare original with fraction and +1 where one another integer fits:

! 2 <= 1.5
2 < 1.5+1

Answer 2

sqrtl return long double in this case your assumption:

no difference between i <= N and i < N+1

is wrong.