Do you mean this is the same sleep data for the same day and asking why the data itself is different? Or are you asking why they look different regardless of the data represented?
You said it’s very difficult to locate relevant information, is there anything specific you’re looking for?
In my experience, the top more detailed view is available when I’m wearing Apple Watch to sleep. The bottom view is just based on other devices (e.g. when I stop using my iPhone and put it down to charge, combined with my sleep schedule data)