Has anyone else seen huge gaps between tracking codes in AA and numbers being reported from Mailchimp or an equivalent? We've consistency seen 10x differences between the two (with our emailing client being higher) and are fairly confident AA is underreporting.
The difference in 2 different platforms is expected. Mostly, it's due to the logic/method each different platform uses and the way they track click-throughs.
For an ex: The email platform may track 1 click-through every time a link is being clicked, where-as, AA is complete;y dependent upon how you have implemented the 's.campaign' variable. In a scenario where, s.campaign variable is using the 'getvalonce' plug-in, only unique click-throughs would be recorded (determined per the expiration defined in the plug-in argument).
As a result of the above, the email platform would report more clicks for a particular code when compared with AA as the AA variable has been implemented to track only unique clicks.
The above is just a random example to help illustrate how numbers for 2 different platforms can differ.