First, I think it's probably fair to say that for every single phone that shows four signal bars, every user everywhere will always see 1-2 bars except where they see 3-4.
The only case not included in that is no bars-- no signal at all. When you say you never see areas of low signal, that basically means that you've never left your coverage area (not that your operator has signal everywhere, because none of them do). What it does mean is that for the purpose of demonstrating this antenna flaw-- whether the apparently dramatic attenuation in the iPhone 4 or the less dramatic drops in other vendors phones-- you lie outside the scope of the experiment, since the entire issue revolves around what happens when that attenuation takes a signal that *is already low* and drops it below the point where it is usuable-- to where it negatively affects call quality or causes a call to drop.
If you never travel to a location where your signal level is that low, you probably won't see a noticeable drop even if your handset had such a flaw and you were holding it in such a way to cause that flaw to manifest.