Array allocates space for more elements than requested

When I execute the following code, c_copy is initialized with more elements than specified by sz, what is causing this behavior? I thought that perhaps it was happening because arrays required a constant value, but declaring a constant value in or outside of the method did not help.

char* e4_strdup( const char*& c )
{
    unsigned int sz{ 0 };
    for ( const char* p_to_c = c; *p_to_c != '\0'; ++p_to_c )
        ++sz;

    char* c_copy{ new char[ sz ] };
    for ( unsigned int i{ 0 }; i < sz; ++i )
        c_copy[ i ] = c[ i ];

    return c_copy;
}