In the case of Drizzle where something similar but more complex is happening, the noise in adjacent pixels is correlated. The result looks visually cleaner and noise estimates are lower, so Drizzle appears to do some noise reduction. From a science POV the noise is still there, of course.
This link has a good description but isn't working right now. I presume it's just a temporary DNS glitch:
http://www.stsci.edu/hst/HST_overvie...34.html#385457
Cheers,
Rick.