Exactly like Pokemon, how the fuck does a fish evolve into an octopus
I know it's been like a year and a half, but we do actually know why for those two. In the original sprites, remoraid was based on a pistol (the current stripes on its back were originally revolver chambers) and octillery was a mix of a soldier and tank (with the lines on its head being a helmet, the mouth being a cannon, and the tentacles being treads). So the line was meant to be a mixture of sea creatures and military.
But they got rid of the military portion past beta (probably didn't want references to guns in the kid's game) leaving us with the strange pairing.