I'm looking at my total posts/replies for the past 7 days in Activity Snapshot but it's not the same number as what is listed on the Post/Replies chart on my Group Activity History. What's the difference?

The past 7 days in the Activity Snapshot section reflects data from yesterday plus the prior 6 days. 

The Post/Replies chart reflects weekly data looking at calendar weeks, Sunday through Saturday. 
Weeks are marked by the "beginning date".  For example, Week beginning 9/20 shows weekly data for Sunday, September 20, 2009 through Saturday, September 26, 2009.

You can find other great tips and ideas about the scorecard in the Groups Resource Center.  Owners and Admins of some of CafeMom's largest and most active groups are in the Groups Resource Center and they have tons of information to share with new and existing group owners.