I have attempted to answer this question through research, but I'm still confused.
Technicals: Moodle 2.2, "Single uploading of file" assigment set to "Separate Groups," "Email alerts to teachers" set to "Yes."
Problem: A teacher is receiving email notifications only for some student submissions, not all. It seems that notifications are being set for students submitting the assignment who are not in any group.
All other notification processes seem to be working fine -- cron is working, forum subscription works, etc., etc. It is only this teacher who has indicated she isn't receiving some notices.
Do I put the teacher in each group? She is the only teacher for the course, so I had assumed she would receive all notifications.