The definition mentions both difference and unity, but in different respects. If subject and predicate were wholly separate in every respect, predication would fail. If they had to be identical in every respect, informative predication would be undermined.
In “A human is an animal”:
Human — the concept of a rational animal.
Animal — a broader genus of sentient living beings.
The concepts are different while every human individual is also an animal. Keeping conceptual difference alongside referential unity is essential.