a) your assumption that it should be distributed normally, was wrong.
b) a rare instance of chance variation; even with n=30, some samples from a normally distributed population can be skewed.
c) the influence of an unanticipated second factor.
A sample of n=30 is never perfectly normally distributed, even if the underlying distribution is normal.
Most of the time, the samples will a show a distribution quite close to a normal distribution. But due to
chance, a certain proportion of samples will look different.