[Serhiy Storchaka]
> ... I want to say that double rounding causes not
> only bias from ideal distribution, but a difference
> between platforms

That's so, but not too surprising.  On those platforms users will see differences between "primitive" floating addition and multiplication (etc) operations too.

There's a tradeoff here.  In Python's earliest days, the bias was strongly in favor of letting platform C quirks shine through.  Partly to make Python-C interoperability easiest, but at least as much because hiding cross-platform fp quirks is a major effort and even Guido had only finite time ;-)  As the years go by, the bias has been switching in favor of hiding platform quirks.  I hope Python 3 continues in that vein, but Python 2 probably needs to stay pretty much where it is.