I speculate that Apple originally intended to use a 
    fancier variable-length encoding, probably based on a 
    Huffman code. This turned out to be too difficult, or 
    not worth the effort, or they just ran out of time, and 
    so they dialed it back to the less ambitious version 
    seen above, where strings choose between constant-
    length encodings using eight, six, or five bits per 
    character. The weird table lives on as a leftover, and 
    a starting point if they decide to go for a variable-
    length encoding in the future. This is pure guesswork, 
    but that's what it looks like to me.

A much simpler explanation is that someone just copied and pasted from the Cocoa string character frequency table.