Ozziebob, it could easily be the last 12 months' tier miles, like in your dashboard, to avoid a drop every time you qualify.

I also heard from crew that it's a combination of status, miles and ticket fare (flex before saver, paid tickets before awards etc.).

There is definitely some liberty where the load controller can decide. I was once upgraded at the gate, while talking to a colleague. He was on the return flight of his first EK trip, not even registered with Skywards. The agent asked whether we were travelling together, and promptly upgraded him too.