And? Characters are not immune to bias, even toward past incarnations of themselves. Something we can consistently track across all of the Sith in the Star Wars Canon is their belief that the dark-side is unequivocally stronger than the light-side. And now let's take Anakin Skywalker, who's power and abilities were blatantly suppressed and restricted by both his own emotions and the Jedi teachings. It makes perfect sense that Darth Vader is going to regard his new self, who is free from any restriction, along with now using a version of the Force that Palpatine touts as infinitely more powerful than the light-side, as more powerful. He's finally raw and free - which Lords of the Sith states, I believe. However, the truth of the matter is that Darth Vader is in denial - just as Dooku was when he claimed the dark-side has made him the most powerful Jedi. George Lucas has gone out of his way to clarify that Darth Vader is a "pathetic character" who's power was vastly hindered by the Mustafar experience. He's also likened Darth Vader's power to that of Darth Maul and Darth Tyranus (of course noting that Anakin Skywalker defeats the latter in battle). Contrast that with Anakin Skywalker, who George Lucas ranks as a combatant on par with Yoda and Darth Sidious, and who's been stated by those who work closely with George Lucas, and thus are obviously reflecting his views, to be the most powerful Jedi in history as of Episode III.
If you want my opinion, I think you are so stubborn on this issue because you think conceding would be embarrassing. But if you think logically about it: George Lucas has stated and shown that the tragedy of Darth Vader is that Anakin Skywalker once had so much and then lost everything. It does not follow that Darth Vader manages to retain his power - especially considering he's more of a machine and thus his abilities are greatly reduced. Again, refer back to what George Lucas said when preparing to Episode I... that Darth Vader was a "half-man, half-robot" and ultimately a shadow compared to what we are seeing in the "golden age of the Jedi" of the prequel trilogy. Arguing that Darth Vader to be more powerful than himself in his flesh prime is a disservice to the story that George Lucas was trying to tell.