Strength and resilience are two different things
Zaraki > Yamamoto.
Zaraki survived being in space.
^ this is one way to look at it.1000 years*
I remember that somewhere was mentioned that Kenpachi is not just title given to strongest shinigami, it's also title given to shinigami that slay most shinigamis/souls. In barbarian way of life, killing most people would also mean being the strongest. To me it has more sense.