I'll try. My understanding is that in x time (here 16.5 months) they will have processed 80% of cases. Doesn't matter if most were done sooner, it matters how long it took them to hit 80%.
So... Let's say there's 100 cases in a month, 80 of those cases will have heard from USCIS *within* 16.5months. That may mean they heard one week after applying, may mean they heard 16.5 months after applying but by 16.5month mark 80 of them should have heard something while 20 haven't
Although it's *at least* 80% so if for some reason they approved a big batch at same time they crossed 80 threshold, they may be over 80%