Why with equip lv40 i can attach only maximum materia II ?? why materia III only with equip 41+??

Not more correct if:
Equip 1-20 -> Materia I-
Equip 21-30 -> Materia II-
Equip 31-40 -> Materia III-
Equip 41-50 -> Materia IV-
?

Pls explain me because i don't understand ;__;