The Summary metrics at the top of the Conversion dashboard count every single "complete" conversion event fired on your site, but not all complete events can be traced to one of the top stories/sections/topics/authors seen on the bottom part of the dashboard.
When we attribute conversions to a story, section, topic, or author value, we look at a user’s reader history before "awarding" a conversion to that value. For a small segment of users, there may not be a prior user history so we're unable to attribute that user's conversion to a particular value. This can happen if a user navigated directly to the checkout page (perhaps from a 3rd-party ad instead of via an article), has blocked cookies on one page but later consented to cookies, or is using an adblocker.
For this reason, we don't recommend summing up the Conversion counts from the Top Stories list to compare against the top Summary metrics.