Ok so now we have seen Rukia's zanpaktou and its similar to Toushiro's. It got me thinking as to how a shinigami actually gets a zanpaktou with that particular ability i.e. Why is Yamamoto's the most powerful attack zanpaktou, Ichigo's one that magnifies his reiatsu, Byakuya's tiny blades etc.

Any thoughts? Or was this explained somehwere and I have missed it?