just a thought. I wonder if Roger was much stronger than WB, in the same way Luffy is much stronger than Kid and Law. Because Kid/Law seem to parallel WB in that they are the only ones who's bounties seem to rise alongside Luffy's.
While Roger did fight "equally" with WB, its clear that they weren't going all out. WB's powers can easily destroy an island, yet the island was clearly in tact by the end of their clash.
Furthermore, when Roger and WB did clash, Roger was terminally ill, WB wasn't. Effectively, dying Roger=WB. And there's decent evidence that suggests prime Roger>WB, as prime Roger took down Xebec.
idk, WB is said to have equalled Roger, but who knows how true that is. By looking at bounties alone, one could easily come to the conclusion that "Law=Luffy" or "Kid=Luffy", even though we all know that that is wrong.