1. They had four jobs, you passed the interview but were 5th or lower on the list;
2. It's bulk recruitment, they have no idea where to place everyone yet so they stuck everyone on a reserve list.
Either way, there's a high probability you will get offered a job. In scenario 1 people often decline offers - the last campaign I did we had 3 posts and two declined the offers so the posts went to the first two on the reserve list. For my current post I had scenario 2 - bulk recruitment and they were sorting out their arse from their elbow for weeks before they made me or anyone else an offer.