It’s impossible to know every assignment on every play unless you’re in the room drawing it up or on the field executing the call. This applies to every facet of the game. We don’t know the audible called at the line and what led to that decision. We can’t know every route adjustment or tag for every scheme from every team. We don’t know how every coverage will unfold, as on many types of coverages, they’re reacting and adjusting on the fly to the route combinations being thrown at them.
Busted coverages make the original play-call even harder to identify.