Jeff Sauro:

“In usability testing, this [Serial Position Effect] is most commonly seen with tasks and product order.
Users typically perform worse on their initial tasks (as they get accustomed to the testing situation and being observed). When testing multiple interfaces or products, the most recently used product may have more salience in the user’s mind when they are asked to select their preference.
The best way to minimize primacy and recency effects is to alternate the presentation order of the tasks and products.”