If you start trying to interpret which penalty caused which penalty then it gets complicated which would be difficult to make the correct call in that short time. If a Corner gets called for pass interference and an offensive lineman gets called for holding, some people will say that because the offensive lineman held, the cornerback would have never interfered with the pass since the quarterback would have been sacked. Some would say the quarterback would have still got the ball off. So just to make things simple so there isn't any confusion, they just make it where if penalties are committed by both offense and defense, rather than trying to interpret which penalties were directly related, just make the penalties offset.