Because the earlier models don't have the antenna built into the case like the new one. I think there's clearly a greater issue with reduced reception with the iPhone 4 because of it.
In retrospect, the "incorrect bars" doesn't sound so far-fetched to me. I either have "full bars" or nothing on my 3GS most of the time. I can't remember ever seeing a 3-bar signal since getting it 5 months ago.
Two friends walked into a bar. The third one ducked.
I read a post here that nokia tried the same thing and eventually did find a fix HOWEVER apple does not want to pay for the right to use the fix.
Instead they want you to buy a “bulk up” phone case.