At the end of every day, every Coy would produce a concise summary of the day's events, according to a standard template. This would include a skeleton of relevant info around each contact report, refined and maybe supplemented with additional 'after action' information.
At every level, the night shift watchkeeper was responsible for assembling the NIREP as per the timetable, and for briefing their commander in the morning.
I did this at Coy, Bn and Bde level.
I'd lay money this is the basis of the estimate calculated by HQNI
That said, you might want to read back to my comments about Bill, and his observations about how (in 1972) how few contacts were actually reported.
Upshot? The official figure is but a Scientific Wild Arrsed Guesstimate (SWAG), and likely a gross under-guesstimate.