I like thinking that he and Fujitora were exceptional anomalies/the strongest warriors in their respective Countries (like Kyros for DR or, better, Kaido).
I don't think that there are many of these monsters around. In fact, I think that from now till the end we won't see any other like them. Fuji ad Ryo were created because the plot needed them; without that, these two wouldn't have existed.
Realistically, Oda didn't even bother himself with a proper explanation.
For some reason, he didn't foresee the necessity to have two strong characters ready to become new admirals at the end of the first part of the manga. When he was presented with that necessity, he created two new characters out of the blue. For example, if he had planned beforehand, he could have said that two very strong VA were stationed permanently in the New World for defence against the Emperors and Rev, so they weren't present at Marienford.
Nonetheless, if I had to imagine a possible background, I'd say:
1) extreme strong anomalies and likely the absolute top warriors of their respective nations, like Kaido, Kyros (not strength-wise, I mean position-wise), etc...
2) very, very famous bounty hunters maybe.
I don't think that there are many of these monsters around. In fact, I think that from now till the end we won't see any other like them. Fuji ad Ryo were created because the plot needed them; without that, these two wouldn't have existed.
Realistically, Oda didn't even bother himself with a proper explanation.
For some reason, he didn't foresee the necessity to have two strong characters ready to become new admirals at the end of the first part of the manga. When he was presented with that necessity, he created two new characters out of the blue. For example, if he had planned beforehand, he could have said that two very strong VA were stationed permanently in the New World for defence against the Emperors and Rev, so they weren't present at Marienford.
Nonetheless, if I had to imagine a possible background, I'd say:
1) extreme strong anomalies and likely the absolute top warriors of their respective nations, like Kaido, Kyros (not strength-wise, I mean position-wise), etc...
2) very, very famous bounty hunters maybe.