PG-13 Horror Movies: Good Horror or Just Horrible

I don't really understand how the rating system works when it comes to horror movies. I realize the rating system was put in effect to keep young adults from seeing things they shouldn't. However I have grown up on horror and as far as I can tell it hasn't had any negative effects on me. The majority of the horror that I watched was R-rated which is supposed to stand for restricted unless you're with an adult I guess. PG-13 meaning 13 or older can get in but here's my question how many kids under 13 are going to go to a movie without their parents. PG meaning parental guidance which I think is self-explanatory. Here's the part I don't understand who gets to decide what is R-rated and what is not. It's my opinion just like everything else the rating system is rigged. For instance the Texas Chainsaw Massacre (1974) is rated R. yet very little of the violence is actually shown. Yet in The Ring and The Grudge which have more grotesque and violent scenes then Texas chainsaw massacre they only received a PG-13 rating.

Every individual finds something different disturbing so this commission or committee who is rating these movies are they going by strict protocols or is it just there judgment call on each individual movie as to what it should be rated. I hope it's the first of the two but after seeing how they rate two different movies with about the same amount of gore and violence with two different ratings I'm guessing it's the latter.