You see its like a puzzle piece.
when the continents first got separated they would fir perfectly into each other. but as time passed the continents broke apart even more forming islands and landmasses that drifted apart and hit against another continent such as India. due to eruption of submerged volcanoes new islands appeared on the surface. certain landmasses got submerged coastlines got indented. further tectonic movements changed the shape of continents by forming mountains. this way today the puzzle pieces are no longer perfect fit but we can guess that they were once together by the similarity of shapes. most common being that of Africa and south america. 
Super continent was scattered into pieces and as time passes those parts undergoes soil erosion and that causes their original shapes change.