There are several different reasons for this. When memory foam is manufactured, there are various elements that can have an effect on the final product from the high or low percentage of humidity to the extreme heat or cold. The first item you purchased may have been produced in the summer with high humidity while the second in the winter in extreme cold. Although the chemistry is modified somewhat to address these elements, it is not a perfect science. Additionally, the retailer could have requested some modifications to the actual memory foam while keeping the appearance of the product and the packaging the same.