Trouble with understanding this margin concept

Step 21 in the Rothko painting has me stumped. I don’t quite understand the added white space (other than it being the top margin, but why does it happen?) on top of .one, nor do I understand the explanation…

Now .one is centered horizontally, but its top margin is pushing past the canvas and onto the frame’s border, shifting the entire canvas down 20 pixels.

Any way someone could break this down to more simple terms?