GSP and Anderson Silva have similar dominating performances against a similar caliber of fighters. The biggest difference I see when rating GSP and Silva is most recent performances. GSP was TKO'd by Serra and lost his belt. Silva keeps dominating everyone put in front of him and still holds his middleweight belt. The supposed near loss to Lutter, who didn't even make weight for the fight, isn't as much a blemish as getting TKO'd by Serra. That puts Silva higher in the P4P ratings.
Perhaps Hendo could take a best of 5 series with Silva, but that is pure speculation. What is known is that Hendo is 0-1 against Silva.
Item 3 places Silva even further up the P4P chart as he beat Hendo, hence he beat a guy who competes at Light-heavy (more pounds for the P4P formula). We have yet to see GSP beat a middleweight and he lost to a guy who usually competes at lightweight (Serra).
Hendo did not routinely defeat men 30+ lbs. larger than him in the Pride middleweight division. He defeated men that may have had at max 15 pounds on him. If he competed in Pride's middleweight division as a welterweight he would have weighed in at 185. A 30 lb. difference would put his competition at 215 lbs which is heavyweight. The only 30+ lbs. difference fight he competed in was vs. Big Nog and Hendo lost.