Had there been three events in that bin, there would have been no surprise. Three is rougly the expected number. The standard deviation is the squareroot of 3, which is 1.7. That means that about 2/3 of the time, we would expect to see either 1.3 events (i.e. 3 - 1.7) or less, or 4.7 events (3 + 1.7) or more.

There are about 12 bins in the histogram. We expect 1/3 of them to be outside one standard deviation. Therefore there should be about 4 bins that appear to be fluctuating away from the expected value.

We expect to exceed 2 standard deviations about 5% of the time. So it is not too surprising to find one bin with zero events, when 3 +- 1.7 is expected. (Zero is within 2 standard deviations = 2 x 1.7 = 3.4.)

**More precise calculation.** The square-root rule is only approximate.
We can do a more precise calculation if we use some results from the theory
of statistics. Those of you who have studied statistics might find this calculation
interesting. If you haven't studied statistics, then this will make no sense
to you, but don't worry -- the square root rule is all you need to become a
future President.

The plot should be well approximated by a Poisson distrubution. In the Poisson
distribution, if the expected number is 3, then the probability of observing
0 is exp(-3) = 0.05. With 20 bins, you would expect one of them, on average,
to fluctuate down by this much. So again, we conclude, the zero bin is not unexpected.