There is a relatively straight-forward answer to the original question of "why the screen is different":
There are a few, different display manufacturers/suppliers in the world. They provide displays with different characteristics like resolution (or manufacture them in any resolution you want, if you can afford the price), number of colours (4K, 64K, 262K, other), brightness, technology used (TFT, OLED, whatever), etc.
For any particular model, or perhaps a "family" of models, a device manufacturer would agree to buy a display (or any other component) with a selected set of features/characteristics from one or more suppliers.
Component manufacturing is then set up to supply exactly that kind of displays.
When a new product is designed/planned, a different or newer display with more or less different features/characteristics can and often is chosen (e.g., same resolution but perhaps brighter from the user's point of view, or any other difference) from the same or different suppliers/manufacturers. Then that display is used in a new product, or several new products.
It could even be the exact same display suppliers as with the older product(s), but they have improved their product and can now be providing, e.g., similar displays as before, but brighter, using less power or just cheaper (to make, buy or both).
In the end, it may result in a situation where a consumer ends up with two different devices in their hand(s), with similar key features/characteristics (like display resolution, number of colours, same technology such as TFT, etc.), but they still look different (e.g., one looks brighter than the other).