98%15 is indeed 8.
15*6 = 90
98-90 = 8
That said, your program still has a few flaws. main is not returning an int, like it should. You are not using stdlib.h yet it is still included, and you are using the depracated header includes (standard headers do not end in .h). Here is a more cleanly written version.
Code:
#include <iostream>
using namespace std;
int main()
{
float float_num1;
float float_num2;
int int_num3;
int int_num4;
cout << "1st number:";
cin >> float_num1;
cout << "2nd number:";
cin >> float_num2;
int_num3 = (int)float_num1;
int_num4 = (int)float_num2;
cout << int_num3 % int_num4 << endl;
return 0;
}