need an explanation of strings/add behavior

[r3d9u11]

Hi. I'm tried to do something like a: 
    
    
    var s = "str123"
    s.add(s)
    echo s
    

I'm expected 'str123str123', but got 'str123str123s'. Why did appeared the last 
symbol 's'? What difference beetween concateration (&) and adding strings?