latin - What is the difference between "Hept-" and "Sept-" prefixes?

It's the difference between Latin and Greek. Four, five and six do have their own forms.

Latin: bi-, tri-, quad-, quin-, sex-, sept-...
Greek: di-, tri-, tetra-, pent-, hex-, hept-...

Generally, Latin prefixes are added to Latin roots and Greek prefixes to Greek roots.

Quadrilateral, quintuplet, September etc.;
Tetrahedron, heptagon etc.