Tie them however you like. It is much faster to tie a fanned wing than any alternative if you are comfortable with the pattern, but I've always assumed the full wing was meant to imitate wings in motion, but I would bet economy of effort was the real reason. Tie them your way and see if they perform differently...my bet is they won't, but experimentation is what tying is all about.
If you are actually trying to ask 'why is pattern X not more realistic', then that's a different thread. None of our patterns are particularly realistic...but that's another thread...
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