Whatever formula they use, those rankings are completely worthless for defensive players and always have been. The fact that DeMarcus Ware is their 34th-ranked linebacker should make that absurdly obvious to you.
Anyhow, those rankings say Roy is the 23rd-best defensive back in the entire NFL. Not the 23rd-best safety -- 23rd-best defensive back. Ahead of Kerry Rhodes (29th), Sean Taylor (31st), Adrian Wilson (48th), LaRon Landry (56th), Ken Hamlin (71st), Darren Sharper (83rd), Ed Reed (95th), Troy Polamalu (169th), Bob Sanders (171st), John Lynch (282nd), Rodney Harrison (301st), Brian Dawkins (315th) and almost every other safety, not to mention almost every cornerback, including Asante Samuel (94th), Al Harris (117th), Champ Bailey (126th), Nnamdi Asomugha (226th) and Terence Newman (239th). In fact, Roy is the highest-rated player on our entire defense.
Are you sure you still want to use those rankings as part of your argument?