And a poor PRNG or poor implementation of a uniform distribution (both extremely common, for example: rand() % max) has more clumps, my question is still valid.

Edit: I wrote a program[1] to fill random pixels based on rand/modulo and mersenne twister/uniform distribution and I could not tell the difference in the generates images.

[1] https://gist.github.com/anonymous/9c726ea636f06900ba3a