I'm a PhD student at a university, and have been given the low-down on how this works from the university's point of view.
Anyone who gets enough points for their firm offer gets in to their first choice... - no questions asked, no means to change things.
Anyone who misses the points for their firm offer is looked at by their firm university, and we decide whether we still want them. If the answer is yes, - their offer goes through automatically from their point of view.
If the first choice university says "No" - then it goes to the insurance offer. If the person has enough points for this, their insurance offer is accepted automatically.
If they haven't enough points for their insurance offer, the insurance offer then decides if they still want the student (in much the same way as the firm university did) - If the answer is "yes" then the student is accepted, if "no" then the student goes through clearing.
I expect the system is trying to tell you that your firm university saw that you'd missed your grades, and decided that they still want you anyway.... Congratulations! - Best of luck with your new life at university :) - If you're really unlucky I'll be one of your tutors.