Shonen rulebook says that Luffys strongest Marine and Pirate opponents will be stronger than Rogers, so Akainu and Teach are guaranteed to be stronger than Garp and Rocks. But Garps strength level isn't exclusive to him, he had Sengoku as a near equal. Akainu can be the same with Kuzan and Kizaru; just like Rogers level wasn't exclusive to him, he had Whitebeard, Shiki, Garp and Rocks as comparable/equal. The only exclusive power level in this story is Luffys, which is why he's going to go above and beyond all these characters-take down Imu, surpass Joyboy and end up ultimately peerless at the pinnacle. Seems like that's what Oda is going for.