I dont know the exact implementation. But following article suggests that there is some lag due to data being exchanged with iCloud.
http://www.cnet.com/au/how-to/macos-sierras-universal-clipbo...
http://www.cnet.com/au/how-to/macos-sierras-universal-clipbo...
[1] http://www.idownloadblog.com/2016/06/23/macos-sierra-preview...