According to the interview with director Shinbo and producer Iwakami, "Bakemonogatari
" will be 15 episode long. The programming block of the TV channel accommodates only 12 episodes, so they plan to stream the rest of the three episodes on the official website for free. Director Shinbo decided on the irregular series organization in order to reproduce the original novel faithfully.
Director Shinbo insisted that the idea of the un-aired episodes was not developed on account of their marketing strategy.
Source: Bakemonogatari official website
Aniplex was surprised to know the enormous amount of money required to borrow data servers for the streaming.
Source: Sendenman blog