Why does UIResponder.keyboardFrameEndUserInfoKey return different values?

Question

iOS

I'm using NotificationCenter to react to the appearance of the keyboard and update the constraints. Everything works fine, but if you start closing the view with a swipe and, without finishing the swipe, return your finger back and release it, UIResponder.keyboardFrameEndUserInfoKey will return a different value than the previous one. With what it can be connected?