According to w3c specs for Box Model
The rendered width of a box type element is equal to the sum of its width, left/right border and left/right padding.
So that means the values of padding and border-width affects the total width of the element. So here you are adding the 15px of the padding along with 5px of borders with 100% of actual width of the element.
So overall it exceed the widow size that's why it comes with horizontal scroll.
