They probably set a high max bid. So say you are willing to bid a maximum of 2000 credits. You would bid that amount and as long as the other persons max bid is lower, you will be the top bidder. If someone bids 1500, you would then have your bid automatically set to a bit higher than that, and you would remain the top bidder. So even if you bid 2000, you may only end up using 1510, for example. Does that make sense?