More fluffier more control

For 2nd serve they're looking for more control than speed to avoid a double fault.

They want to make sure the ball does land in.

For this they choose the fluffier ball, it gives them a much firmer grip.

It's not like you are going to win the match because you chose a fluffier/ less fluffier ball but there's science behind this and you should use that to your advantage.

Fun fact: Serena Williams is the only player who doesn't pick a ball, she plays what she gets. One at a time.