Maybe Roger actually did eat the Gomu Gomu no Mi.
Look at it this way. Oda's greatest idol is Toriyama, the guy who created Dragon Ball. Well, in Dragon Ball, after a wish is made, they get seperated around the world.
Well, perhaps when someone with a DF dies, the fruit gets sent to some random part of the world.
This would explain why Shanks went to the east blue in the first place. He heard word that a fruit had been found and went after it. There aren't many DF useres outside the Grand Line, so it wouldn't be that bad of a chance of the fruit actually being the Gomu Gomu no Mi.
But, when Shanks made it to the East Blue, he discovered that an enemy ship had gotten the fruit. So Shanks stole it back. That's why he said "We got that from an enemies ship!"
Anyway, yeah. Why Shanks would want to get Gol's old fruit? No idea.