Well, it's kinda like a game of telephone or a fairy tale. The Celtic cross is created, passed on person to person. Things get mixed up (usually toward the end there), and finally when people write it down, it gets written down 3 or 4 different ways.
Spreads are personal things. Whatever works, whatever feels right, do it. When you get a piece of music, for example, it's the same piece of music for 3 or 4 people, but they'll all play it their own way, with their own flouish and extras and interpetations.
