Hi. Without parenthesis you see the real number of messages coming from the decoder. Between parenthesis you see the condensed number passed to the application for processing. RadarBox software, prior to working messages, tries to condense them in order to improve application performance. SO if for example, one flight sends 7 different messages in 1 second (one with altitude, the other with V/S, etc), the application adds all these to single message so the master database is only edited one time (better efficiency and performance).
Hope this explains it.