The tackle or pass defender is randomly assigned within all defensive groups. So, if you are defending the deep pass to a WR, then the output on the play by play is randomly assigned to FS/SS. If a player breaks through the primary defensive group, the next closest group gets credit for the tackle (if made) and also randomly assigned.
This is why sometimes you will see a deep pass receiver sometimes tackled by a LB (or defended by a LB), since the LB group is the next closest group if WR are on short routes (thereby making the CB stay close to the line of scrimmage).
You should note I used the term "defensive groups". The random assignment of the tackle for example on an inside run is assigned to the defending group defending that play (which includes DL/ILB).
There are some threads on the assignment of sacks that is interesting if you want to pump up a players sack total if that twirls your ponytail FYI.
nitros (james)