I placed a bid but I'm not the leader

This is caused by your attempted bid "pushing up" another bidder's proxy bid.  For example, if a unit is at $40 and somebody bids $100, they become the leader at $50.  If you then try to bid $60, you'll push up that bidder's proxy bid to $60 and they are still in the lead.  When this happens, the auction page will show a message that a prior bid existed.