virgindriver, I have absolutely no idea why it is done this way. I don't control seniority, it's a company and AIPA thing.
I will say what I have said previously and that many cadets previously (myself included more than a decade ago) joined QF without industry placement. No one (yet) joins QF and goes straight to F/O. I knew of someone who was senior to me who joined after I was an F/O (she did her obs flight with me) and she was still a S/O for about a year (or more!).
I also have no problem with a GA driver being told by QF 'yep we like the cut of your jib and we look like starting you on the mainline fleet in a couple of years time but first we want you to go and flog around with Air North for a couple of years but if you stuff it up then we may not take you after all' and being given a seniority number in the same manner that the cadets do.