First of all: Sorry for my bad English. I am writing from Germany.
I suspect that every iPhone 13 (Pro) has this gap. In our company we received eight iPhone 13 Pro (Max). ALL (!) Have this gap and you can see two pieces of metal. If you look at videos in which the iPhones are being disassembled, you can clearly see that these pieces of metal are there to hold the new speaker in place. The display is in the frame, so the gap is normal. But depending on how accurately the display is embedded, there is a larger or smaller gap at the top. Then you can see the metal.
With my iPhone I don't see it at all in normal light. Only when the sun is shining directly on the iPhone and I am looking at the iPhone from a certain angle from the front. That is why I am convinced that the YouTubers' devices also have this gap. The light and camera just never show it so frontally and at an angle from which you can see it.
With this YouTuber from France you can see it very clearly - with corresponding frontal light:
I went to two Apple stores over the weekend. The light in the Apple Store is so "soft" that you can't always see the gap. But as soon as you light up the notch with the flashlight and change the angle a bit, you can see the metal. Sometimes more sometimes less.
With our eight iPhone 13 Pro (Max), the gap is sometimes larger, so you can see the metal even without direct sunlight. So it's pure luck how much manufacturing tolerance you have. Now that I have seen over 20 devices, I am convinced that every iPhone 13 (Pro) has this cosmetic "problem". I've also seen it on an iPhone 13, but in general the aluminum frame seems to close better to the display than the stainless steel versions.
I'm tired of playing that Apple lottery. My iPhone 12 Pro Max last year had similar issues - but at the bottom. There I also saw two elements made of metal, which probably fixed the Lightning connector to the housing. I had a total of 8 (!) Replacement devices, so I stayed with one device that was acceptable to me. After two weeks you don't think about it anymore in everyday life. Still, I find it annoying how badly Apple's quality control fails year after year - when we, as "normal" customers, discover these errors after unboxing.
The iPhone 11 Pro had sharp edges on many devices, and the iPhone XS Pro had a cracking sound when the device was moved back and forth a little. And so forth ...