Why don't they present shape of the continents fit perfectly into supercontinent

The edges of the continents are in a constant state of change caused by the ocean currents. Those currents can bring material with them, making the line more "smooth" and adding land to it, forming the so -called barrier beaches, or the sea can cause the land to fall into the water.

this change of edges over time and tectonic movement mean that the shape of the continents changes all the time: so the continents changed since the time of the split.