It's usually very slight, but it's there. It's the difference one can often only see by comparing the AF result with the human adjusted correction in MF. Sometimes it's bigger and can be seen in the files but this could easily be mistaken for a bit (1 pixel or so) of camera shake or just a naturally soft lens. So if you're not checking by MF you're probably just thinking that's how it's supposed to look.
But we're asking the camera to both do the impossible by reading our minds and also go beyond it's physical limits too. It can't actually read our minds so for one example when the AF box is the size of our subject (say, a coffee mug on a table) selecting which part it should focus on (the front, the upper lip, the lower seat, the outline edge, the handle, the rear rim lip) is just random chance or maybe the part with the strongest contrast. Same with a face when not in face detection mode. It won't know if it should focus on the tip of the nose or the stray hairs from the pony-tail in back. The AF systems in the GH1 and 2 are extremely limited and can almost NEVER achieve "critical focus". This is well known by many.
It does also sometimes get the entire shot (or mug in our previous example) wrong. This is true with the OM-D, 3 different GH2s that I've used and a countless (8 or 10) GH1s - also several of the other Olympus µ4/3 models too. It's just the nature of the beast. If you never half-press plus re-half-press your shutter release button when it doesn't focus satisfactorily the first time you will get a large number of images which are out of focus - mostly only slightly but sometimes enough to be bothersome too. These count as misses even if you didn't go ahead and capture the image the first time.
Then there are the mechanics of the DOF range which compounds the errors mentioned above. If you're shooting with a typical µ4/3 zoom or any of the slower-ish primes there is a huge margin for error between where the optimal focus setting actually is and what the AF system comes up with. Any of these alone or combined may be within your own personal tolerances but it's not perfectly accurate. In my case 4 out of 5 AF attempts are outside my personal tolerance levels and noticeable to me. So much so that I usually prefer to shoot adapted mechanical manual-focus lenses.
The way in which a DSLR usually works is different. The AF areas (points!) are smaller and MUCH more precise. On consumer grade DSLRs there are usually a lot less of these points which leaves a lot of margin for error in between them. So it is in concept and so it is in practice. Thus my distinction between consumer grade and pro bodies.
No AF is perfect. Pro body AF can often be very good but still has faults. Consumer grade DSLR AF and µ4/3 AF is problematic for all but the most casual styles or high personal tolerance levels. That the GH3 might be better than the GH2 and I would hope the OM-D as well, is good news IMO. However, I won't be expecting a perfect AF system on any camera anytime in the foreseeable future. Unfortunate but true I'm afraid.
