Here's why:
* Kingdom: The broadest category.
* Phylum: A major division within a kingdom.
* Class: A subdivision within a phylum.
* Order: A subdivision within a class.
* Family: A subdivision within an order.
* Genus: A subdivision within a family.
* Species: The most specific category.
Think of it like a hierarchy - you can't be in the same class unless you're also in the same phylum!