In most analytics reports, we summarize lists based on the total number of some value such as shares, particularly if that list would otherwise be too long to display.
For lists of Top Content, we maximize performance by counting every URL that appears more than a given number of times each day in the selected date range.
If a page does not appear within this top number for a given day in the date range, that day's shares are not counted toward the total.
This sampling strategy can sometimes result in slight differences between counts, particularly for very large numbers of shares. The content detail report will always display the correct total number of events for a given time period.