Listia uses a proxy bidding system, which means the system only bids as much as it needs to in order for you to be the top bidder, up to the limit you placed.
For example, say another user places a bid of 5 credits on a listing you want. If the listing starts at 0 credits, the top bid will now be 1 credit since that's all it takes for the other user to be the top bidder. If you now place a bid for 3 credits, the other user would still be the top bidder at 4 credits because their maximum was set to 5. If you placed another bid for 10 credits, you would be the top bidder at 6 credits since that's all it takes for you to be the top bidder.