You can load balance across all members in a pool or you can load balance to members in the highest priority.
Traffic will be load balanced among members in the highest priority, if the member is less than the priority group activation number, the traffic will throw to members in the next highest priority group.
dvwa1 receives traffic because it is in the highest priority group and the group maintains the minimum number of member.
When dvwa1 is unavailable, the traffic is sent towards the member of the next highest priority group.
Despite dvwa1 is back online, traffic still sends to dvwa2 because persistence is turned on for this virtual server.