As you can see in the video below, when a pop-up (or any other element) starts with a slide animation, it starts with 10% of it already visible. It is also more noticeable when it closes. When a Slide out is executed, it does not continue to the end. The animation ends while 10% of the pop-up is still visible.
As you can see, the element will be transformed from -100% to 0, which means that it starts at the beginning of the end location. Which, in your case, looks like 10% that you mentioned.
