Is forprimestep() using Eratosthenes in some modes?

[Ilya Zakharevich <nospam-abuse@ilyaz.org>]

I cannot find any situation in which forprimestep() would be more than
20% quickier than forstep() in combination with isprime().  This means
that I cannot find any range of parameters where the implementation
would use Eratosthenes sieve (as it often should?).

What am I missing?

Thanks,
Ilya