C41 B&W films do not behave the same as conventional B&W films with respect to development. Unless you have a custom pro lab process your C41 film (whether colour or monochrome), it will get the same development time; there is very little push or pull processing of C41 film done.
It is simply a characteristic of C41 monochrome film, whether Kodak or Ilford, that exposing at a higher EI will produce higher contrast negs, all things being equal. My suspicion is that this is because the films have a long shoulder, i.e., are able to record a long tonal scale in the highlights without totally blocking up. Thus, with LESS exposure (higher EI), you get less than minimal detail in the shadows, so they go black and thus overall contrast is increased. As I said, that's my suspicion and I am open to any other information that would correct me.
Other factors are scanning and post processing which can obviously affect how images display on the web.
Earl