Why are Erlang B and Erlang C formulas different?
The basic, qualitative reason for you to have a higher probability of delay is that there are more potential scenarios for delay than there are for simple blocking.  

For example, suppose that all channels are used, and I make a call.  1 minute later you make a call.  In the B scenario I am blocked.  Perhaps a channel frees up in the next minute for you (probability p), perhaps not (probability 1-p).

In the C scenario if I hold for the minute of delay you will be delayed even if a channel comes through, since it would go to me (probability of not delayed p minus something instead of p).  Obviously that's pretty specific, which is why (especially with alot of channels) the difference is not overly large.