Alright, here's a thought: what if Rocks had a Road Poneglyph, and the Marines stole it to hide it on God Valley? Whitebeard ends up with it, then leaves it in Fishman Island, his territory, for safe keeping, and because he wasn't really interested in it.
The Man Marked by Flames stole the Poneglyph under the direction of the WG, and they decided to have him sail around the world with it to keep it safe and hidden.