p(z_i) means the possibility of the value of z_i

if uniformity is very high, for example, 99% of intensity is the value of 150, and 1% of the intensity is 10,

p(150) = 0.99 and p(10) = 0.01.

So, if do the calculation

the uniformity is

0.99^2 + 0.01^2 = 0.9802