Both GSP and Anderson have had stupid upsets in their career, and both managed to bounce back. GSP has avenged his only two losses soundly. Also, I believe that GSP has faced MUCH tougher competition in his division than Anderson has in his.
It's true that GSP has had a number of fights go to decision, but he absolutely dominated his opponents in those fights. Anderson got completely manhandled by Sonnen. That has not happened to GSP. Also, let's look at the stats. GSP has finished 62% of his fights (38% decision). Anderson has finished 75% of his fights (25% decision). So, there isn't a huge disparity in terms of finishing fights.
All that said, these P4P rankings are very subjective. The best way to settle it, is to get these guys in the cage!