I think that it's an alternate semi-evolution of Shellder, and that the change in its typical evolutionary pattern was triggered by a clause of sorts in its instincts/genes.
public class Shellder;
public static void main( String args[] );
if{attached to Slowbro
then{evolve to Spiral Shellder
else{evolve to Cloyster}}}
XD. Everything in the Pokemon universe can be explained in code. :P