As far as I know (probably the only thing that I know), the old randomization has something to do with pattern repetition. As mentioned by Nonak there, the pre 1.6 randomization used a different type of randomization. I know a thing or two about programming, and there's actually no such thing as true randomization. Every randomization used a seed to generate randomness. Even "the most random" randomization still use a seed, so it's basically still predictable, but the 1.6 version onwards used a harder-to-predict (probably can't be predicted) randomization.
Sorry for my bad English.