I can't imagine what this could be. It seems too low for GE/Nexus and way too high for refused entry. What is it...?
From the definitions page:
Excluded: The actual number of passengers on flights which arrived in the selected hour who were excluded from wait time reporting.
That sounds like passengers who were allowed entry but did not need processing time, for example infants who have no passports and share the same passport with their moms. If they were counted among the processed passengers then the report would not be accurate because only one person was passport/person was processed in reality. Just a guess.
One more thing, I do not think they are passengers who were refused entry, because even they need "processing" time and can not be excluded from the waiting time report.