In the example, the button’s containing content element also has padding applied, so there is some breathing room between the edge of the device and the buttons.
The block buttons are shorter in the css page demo because the parent container has padding applied.
Full Width Block Buttons
Adding button-full to a button not only applies display: block, but also removes borders on the left and right, and any border-radius which may be applied. This style is useful when the button should stretch across the entire width of the display. Additionally, the button’s parent element does not have padding applied.
In that example on the css page, the parent container does not have padding applied.
If there is still some confusion, please put together a codepen where you are getting this.