Why different turrets for razor backs and predators?

[Sword Brother Adelard]

When the current plastic Predator was released there was a cross-section produced for a poster in White Dwarf which showed how much of the turret assembly went into the hull. They also have to have space for the ammo/power generators for both the turret and sponsons.

 

Spikey Bits has an awful quality scan of it here: https://spikeybits.com/2020/05/retro-40k-posters-cut-away-land-raider-predator.html