Typically, the reason for the Unique Click mismatch is filtered activity. Account Engagement will automatically filter clicks that it deems were not performed by the prospect - clicks caused by bots or over-aggressive spam filters, for example, are filtered. The number you see on the main report page is the number of filtered unique clicks, where as the number you see when you drill down is the number of unfiltered unique clicks.
It could also be caused by deleting prospects. When a prospect who clicked a link is deleted in Account Engagement, the unique click reporting number will not change (as it still accurately reflects the number of clicks that happened) but the prospect would no longer show in the reporting table.
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