I moderate a google group. People interested in the group sometimes request a subscription. When I go to the web approval interface, I am not offered a choice of delivery mode for the member. After I approve the member, I go to the list of members and check what delivery mode was assigned. It varies. I don't see any way of choosing a setting to control this.
Nor do I see any way of choosing a setting in my own google account to control the default delivery mode for future requests I might make for joining groups.
Edit: here are the four delivery types:
Is there a die being tossed behind the scenes?