User blog comment:Xilinoc/Ch. 540 - The Sword Five/@comment-1043846-20130607042928/@comment-4484409-20130607052305

Well of course you've never heard of a "Shinigami whose Zanpakuto is a Hollow", it's never happened before. Hence the whole "special birth of Ichigo". The hollow fused with his Shinigami powers and now they are one and the same, which is why the Hollow is Ichigo's Zanpakuto. The whole Shikai/Bankai thing has a lot of theories floating around but one thing that seems most plausible IMO is that Ichigo's weapon was partly a Zanpakuto due to his inherited Shinigami powers and could therefore achieve those stages.