Why don't the present shapes of the continents fit perfectly into the supercontinent?

The reason why the shapes of the continents as of today do not perfectly fi into to super continent because long time ago the super continent was scattered into pieces. And as times go by, those smaller parts of super continents undergo soil erosion caused by the water and other way that causes their original shapes change.

Thus, if we are going to judge now if those smaller part of super continent, they’re no longer gonna fit perfectly because through time their original form changed