Actually there is a difference in exposures for both pictures, the N95 is under-exposing the capture while the N900 is exposing it properly, and as a side effect of trying to bring more detail out of dark zones you'll see more noise. My guess is the ISOs are different for each capture, look in the metadata of the pic. Also try setting the ISO manually to 100 and do the test again.

NOTE: I don't know the very specifics of both cameras but even if the ISOs are the same there is no guarantee that the levels of exposure are going to be the same:
http://blog.dpreview.com/editorial/2...ensitivity.htm

edit: Also look at the aperture value and exposure time.