I stumbled upon this page thread as well as a couple of others, before buying the iPhone, serching for "defects" to look for while I am in the return period. I know, I'm not sane.
I've already checked and come to a conclusion, which I've already posted here:
The water tight seal is actually stuck to the glass underneath the display so the small gap you see is not a weak spot for water ingress. You can see this by looking at tare down videos. There is probably a gap on most either at the top or bottom some might be more noticeable than others but...
forums.macrumors.com
Here is my 2 cents:
If you shine a flashlight or look closely at a certain angle in the sunlight, you can see the metal piece above the speaker. It's a construction tolerance. Some units are more affected than others (0.1mm more could make the difference given how tiny the gap is), some units may also have the metal touch the glass and honestly I would be more scared about that, should the phone fall on the ground...
If you ask me, it's not a defect by any means, it's just how it is.
Accept it or exchange it until you find a unit that you can tolerate.
Personally I care more about the display uniformity.