I ran a Clickthrough Summary Report and a Message Performance Report. The Clickthrough Summary report has a much higher number than the Message Perfomance report. Why?
The Message Performance Report is showing you the count of unique clickthroughs from those messages, so if someone clicked on the link a hundred times, it would still only count it as one.
The Clickthrough Summary report is showing you the total number of clickthroughs, thus someone may have gone to a link multiple times from within the message, and it counts each and every instance, so if you have a crazy clicker that likes to open and close windows and revisit pages again and again, that could increase your numbers in the Clickthrough Summary Report. To summarize, the Message Performance Report's number is more exact in terms of unique clickthroughs, while the Clickthrough Summary Report is showing you how many times the page was viewed from the message.