I understand your point. Yes - if there is GROUP A which have NEWSLETTER A and NEWSLETTER B and you subscribe for GROUP A(note that you subscribe by design only to groups) you will receive both newsletters.
I will describe in more details for Groups and Newsletters in Sitefinity.
The subscriber can be subscribed only to a Group (this is by design). The newsletter is like a single mail - it contains the mail body ,subject. By subscribing to a Group you will receive all newsletters(mails) added to that group. Think for the GROUP A like your newsletter and every added Sitefinity's newsletters (NEWSLETTER A and NEWSLETTER B) like new content(mail) that should be sent to the subscribers of the group.
I hope this information will be helpful.
the Telerik team