Because the inner screen is a different shape, not just a bigger one. It is 626 × 890 points at an aspect ratio of about 1.42, where iPhone 18 Pro is roughly 2.17 — much taller and narrower.
Layout code written against the older proportions produces predictable symptoms: content pinned in a narrow column, media letterboxed into a band, or text clipped at one edge because the app assumed both edges were inset equally.
If you are a user seeing this, a plain-language explanation is here. If you are the developer, each symptom maps to a specific fix in the checklist.
The arithmetic
A standard iPhone screenshot is about 2.17 tall for its width. The iPhone Duo inner display is about 1.42. Scale one to fill the other and you lose roughly a third of the image; fit it instead and about a third of the screen sits empty.
Neither is acceptable in a shipping app, and neither is necessary — a layout that reflows pays neither penalty. That is the whole argument for doing the migration rather than scaling your way out of it.
Symptom to cause
| What you see | What the app is doing |
|---|---|
| Narrow column, empty space beside it | Layout pinned to a fixed phone width |
| Video in a band with thick bars | Fixed 16:9 media on a much squarer display |
| Text clipped on one edge only | Assumes both edges are inset equally |
| Something important on the crease | Has not accounted for the hinge |
| Control hidden behind the camera | Drawing under the camera cutout |
A plain-language version for non-developers is on why apps look wrong on iPhone Duo.