Hi Erik,
I think it would be easiest to explain if I share a hypothetical (or in our case, the actual) scenario. We ran an email campaign using a Smart Campaign. A couple weeks later, we did some data cleansing by deleting all leads from Marketo that did not have an email address. We then visited our email performance reports for that campaign, and saw that the numbers were not matching up with what was coming up in a smart list with the same filters.
After quite a while of troubleshooting, we realized we have to hit refresh for campaign members in each Smart Campaign that was used in the report. Once we did that, all of the numbers matched up again.
Huge flaw in Marketo if you ask me. Completely defeats the purpose of the Analytics tab, and as every modern marketer knows, without statistics, you have nothing.