Because now you are making ['thecat'] the same as ['the', 'cat'] (where the second should sort before the first). If we know we're dealing with plain text, then joining with a null character would be ok. If its unicode, I'm not sure...

Another map sort map solution would be to use sprintf to concatenate the elements using fixed lengths, but that would require first knowing what the maximum length of any field could be, and making sure your "%Ns" format is at least that large.