Difference between c++ string append and operator +=

In Microsoft STL implementation, the operator += is an inline function, which calls append(). Here are the implementations,

• string (1): string& operator+= (const string& str)

basic_string& operator+=(const basic_string& _Right) {
    return append(_Right);
}

• c-string (2): string& operator+= (const char* s)

basic_string& operator+=(_In_z_ const _Elem* const _Ptr) {
    return append(_Ptr);
}

• character (3): string& operator+= (char c)

basic_string& operator+=(_Elem _Ch) {
    push_back(_Ch);
    return *this;
}

• Source: GitHub: Microsoft/STL

According to the standard concerning string::op+= / online c++ standard draft, I wouldn't expect any difference:

basic_string& operator+=(const basic_string& str);

(1) Effects: Calls append(str).

(2) Returns: *this.