Your comment is quite interesting! I did not receive only my own contributions, but got fairly large files that seem to me to be exhaustive archives of all posts and other materials uploaded by any and all group members during the entire time I was a member of each group.
It's pretty interesting if different people are being sent totally different things though as you indicate. Maybe I am just lucky.
I'm pretty happy myself that they have this method to get everything from the time I was a member of any group. I can't think of any other service that provides for that. Yahoo seems to be the most open and data accessible service I can recall. Other boards facing imminent shutdown I had to go to a lot of trouble to scrape old posts using bot scripts, which was inconvenient.
Additionally, the 'Get My Data' only gives you access to all the files / photos that you uploaded to the group. These archives should not be considered complete archives of the group - is it really reasonable to suggest that in order to completely back up a group, every member must complete their own data request?