Roger was pirate for Decades before God valley incident, we saw young Garp Sengoku & Tsuru as marines, they were practically friends by the end Roger & Garp. I believe Roger pulled ahead of Garp in power. Just like Luffy in last arc, 3 major powerups in a week lol. Not that big of gap but still, Roger says to Garp, we almost killed each other, that doesn't mean Roger=Garp, Smoker almost killed Luffy multiple times. Roger mentions in Oden flashback, for marines to send Garp or Sengoku to make it INTERESTING, that doesn't mean Sengoku or Garp=Roger.