Sensitivity differences in cameras is REAL!!!
 

I started talking about this back when I had my Pentax K10D. It (and
my later K20D) were at least 1 stop slower than my Olympus gear and
0.7 stops slower than my Nikon gear. Lots of people didn't believe
it, but there was no way around the fact an f4.0 lens setting on the
Olympus gave the same result as f2.8 on the Pentax. It appears now
the OM-D is less sensitive than the G3 (and likely other cameras)
which I know itself is less sensitive than my old G1. So the result
is that noise levels are NOT directly comparable based on "like"
exposures and apertures.
This, from Dpreview:

By R Butler (41 min ago)

The cameras are middle-grey matched. We think that showing you what
the results look like when correctly exposed is far more useful that
showing one camera under or over-exposed, since most people try to
shoot correctly exposed images, rather than insisting on using a
specific shutter speed and aperture.