As you move the slider from left to right the value of np changes from 1 to 70, or thereabouts (Java is an inexact science, at least to me at this point, and you may have to click the slider once to initialize the graph). We're interested almost entirely in the Poisson distribution for small values of np. Notice that they're distinctly skewed to the right, and as a consequence the areas of right-tails (which are the probabilities of finding a z-scores beyond a certain value) are higher than they are for the Normal distribution. We'll look at this distribution in a bit more detail with a class project covering a variety of statistical notions - something to look forward to.