Why does "The Central Randomizer" work much better with non-integer seeds?

From my research, this widely-used pseudo random number generator (http://www.honeylocust.com/javascript/randomizer.html) gives very regular, deterministic results when seeded with an integer (immediately visible, for example, when generating an image with random-colored pixels), but high quality results (completely invisible) when seeded with a real number.

Can anyone explain the scientific background?