The output is not what i expected

Question

I am currently using Microsoft Visual Studio 2019 to compile these code below

#include<iostream>
using namespace std;

double hamTinhTienDien(int kWH)
{
    if (kWH >= 0 && kWH <= 100) return kWH * 1242;
    else if (kWH >= 101 && kWH <= 150) return 100 * 1242 + (kWH - 100) * 1304;
    else if (kWH >= 151 && kWH <= 200) return 100 * 1242 + 50 * 1304 + (kWH - 150) * 1651;
    else if (kWH >= 201 && kWH <= 300) return 100 * 1242 + 50 * 1304 + 50 * 1651 + (kWH - 200) * 1788;
    else if (kWH >= 301 && kWH <= 400) return 100 * 1242 + 50 * 1304 + 50 * 1651 + 100 * 1788 + (kWH - 300) * 1912;
    else if (kWH >= 401) return 100 * 1242 + 50 * 1304 + 50 * 1651 + 100 * 1788 + 100 * 1912 + (kWH - 400) * 1962;
}

int main()
{
    int kWH;
    cout << "Nhap so dien tieu thu: ";
    cin >> kWH;
    cout << "So tien phai tra la: " << hamTinhTienDien(kWH)*1.1 << endl;
}

When i put 251 as the input, the output is 399452 but it is supposed to be 399451.8 I dont understand why . Please help!!!