Activity Map Pageviews dont add up correctly

Hi,

I was calculating the external referrals on our homepage in the page flow display in activity map and noticed that the page views don't add up correctly. (see picture below)

1. I compared total external referrals (3,266) with those within it (3,600).
   1. Question: How can you have more page views in the drill down details (typed/bookmarked, google.com, internal referrals etc...)  then in the top bucket (external referrals) totals?

thanks in advance,

Scott