The mystical world of Staff Travel!!
Think it depends on the mood of the person. I've managed a couple of times to blag an upgrade on my own airline, most of the time I'm sitting down the back (Sure they'd put me in cargo if they could!), other times I've been lucky they've let me on cos I wanted to catch an earlier flight. Although my sister travelling on my concession has been upgraded to club a couple of times.
People I've spoken to from other airlines have told me that usually other airlines will get put down the back.
I think it's just down to luck of the draw. That is the magical way Staff travel works. Dress smart and try your luck