The number shown in the click portion of the email's reporting overview is just for a unique click within the email. The number shown for the links drilldown are the unique clicks per each link.
The difference is :
Click rate: among the openers of your email, how many clicked at least one link.
VS
Click-Through Distribution: among your links, which ones got clicked at least once by a clicker contact.
For more info on how click reporting works, make sure to check out our main article on the topic.
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