Amazon adjusts your bid in real-time (by a maximum of 100% for top of search, or 50% for other placements) when your ad may be more likely to convert to a sale, and lower your bids when less likely to convert to a sale.
For example, if you bid $1.50 on a keyword and Amazon rates the purchase probability as low, the bid can be lowered to $0.40 for example, or even to $0.