Why is it displaying 13 and not 15?

This is the unary operator problem I am facing, and I fail to understand why VSCode gives output 8,13 when turbo c gives out the answer 8,15?

#include<bits/stdc++.h>
using namespace std;
int main()
{
   int a;
    a = 2;
    cout<<++a + ++a;              //This is displaying 8 which is understood
    
    a = 2;
    cout<<endl<<++a + ++a + ++a;  //Why is this displaying 13, shouldn't it be 5+5+5=15?
}