Hello,
This is completely normal. It's called Reachability.
Reachability
Reachability helps you more easily interact with items at the top of the screen on iPhone 6, iPhone 6 Plus, iPhone 6s, iPhone 6s Plus, iPhone 7, and iPhone 7 Plus.
Bring items within reach. Go to Settings > General > Accessibility, turn on Reachability, then lightly double-tap the Home button to bring the top of the screen into reach.
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