Typically when you view the Table format for an event, you'll see data that adds up to a Total.
However, when you split those users by clicking "Add Split", you'll see a Unique User Total instead:
Note that this only happens for Users, not Occurrences.
While the difference is subtle, this feature allows you to view unique users when you otherwise might double count users in the split.
If you export a CSV of the Split By report, you'll notice that summing the column does not add up to the Unique user total since the CSV includes the row values only.