Array concatenation behaves strangely with += shortcut

[Zichqec]

OnTest
{
    _let = ("A","B","C")
    _num = ("1","2","3")
    
    _let = _let + _num
    _let
}

↑ This function outputs "A1","A2","A3","B1","B2","B3","C1","C2","C3", which is the desired output.

OnTest
{
    _let = ("A","B","C")
    _num = ("1","2","3")
    
    _let += _num
    _let
}

↑ This function outputs "A123","B123","C123" instead.