(a) Using Poisson statistics, the margin of error is the
square root of the number of data points.  So if the m.o.e.
is 5%, then the total number of data points is X, where

  sqrt(X) = 0.05 * X   and the answer is X = 400.

(b) A short list of the many possible factors include:
   -- The people sampled do not represent a fair sample of the
      population; gender, political bias, ethnicity, economic 
      status, geography, age, etc.
   -- The question asked may have been unclear or worded to elicit
      one answer preferentially, by accident or design.
   -- Respondents may have felt pressured to pick a particular
      answer, for whatever reason.
   -- Respondents who were asked to participate in the poll but
      refused may not have been taken into account properly.
   -- Respondents may not have seen the entire debate, or may
      have observed the debate differently, for example on TV,
      on radio, or in the live audience.