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Classification exists to scope controls. Encryption at rest, DLP inspection, retention enforcement and access review all cost per terabyte, so knowing that a quarter of the estate is confidential or above turns an unbounded programme into a sized one. The public share matters equally, because data that needs no protection should be excluded from expensive controls entirely.
Classification volumes
public share = 1 - restricted - confidential - internal; encryption scope = total x (restricted + confidential)
Encryption at rest is cheap enough that many organisations do, but key management, DLP inspection and access review are not. Those are the controls classification actually scopes.
The remainder is treated as public. In practice a large unclassified remainder is the real finding, since unclassified data receives whatever protection its storage location happens to provide.