They are obviously in the same ballpark. Unless Garp wanted to fight Shiki for days and destroy Marineford in the process, there was no reason not to make it a 2 on 1.
It's not like we are saying Garp solos 3 Yonko, we are saying the fact that a Whitebeard who wasn't more or less in his prime than Garp (similar age + old Garp showing way better feats than God Valley Garp) immediately decided to back off while backed up by a 30 year old Linlin and Kaido (though kaido as a 18 year old without his DF has the best excuse) kinda speaks for Garp in a direct 1 on 1 comparison, even if the gap is small. as @ZenZu phrased it, saying Garp is at least = Whitebeard based on recent portrayal would have a solid ground.
It's not like we are saying Garp solos 3 Yonko, we are saying the fact that a Whitebeard who wasn't more or less in his prime than Garp (similar age + old Garp showing way better feats than God Valley Garp) immediately decided to back off while backed up by a 30 year old Linlin and Kaido (though kaido as a 18 year old without his DF has the best excuse) kinda speaks for Garp in a direct 1 on 1 comparison, even if the gap is small. as @ZenZu phrased it, saying Garp is at least = Whitebeard based on recent portrayal would have a solid ground.
Garp sits at the very top of the pecking order. Some can join him at worst, he's not below anyone (mayybe Xebec).