Or perhaps better explained in their T&Cs:

For Auctions where you can place a maximum Bid, the system will place bidding iterations on your behalf until your maximum Bid is reached. If you have placed a maximum Bid (higher than the required minimum Bid), the system will automatically increase the Bid on your behalf, using only as much of your maximum Bid as is required to keep you in the winning position. The maximum Bid is the highest price in Skywards Miles you are willing to pay for the Listed Item. If at the conclusion of the Auction not all of your maximum Bid was necessary to win the Auction, your final Bid will only be the amount of Skywards Miles necessary to be the highest bidder and this amount will be automatically debited from your Emirates Skywards account.