Purpose of DEFERRABLE _and_ INITIALLY DEFERRED foreign key constraint checking?

This controls whether the constraint can be deferred. A constraint that is not deferrable will be checked immediately after every command. Checking of constraints that are deferrable can be postponed until the end of the transaction [/quote]
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But yet a DEFERRABLE FK constraint in a transaction immediately failed on a FK constraint violation.

[quote]

If a constraint is deferrable, this clause specifies the default time to check the constraint. If the constraint is INITIALLY IMMEDIATE, it is checked after each statement. This is the default. If the constraint is INITIALLY DEFERRED, it is checked only at the end of the transaction.  [/quote]

INITIALLY DEFERRED solved my problem.  Why do both clauses exist?

(A naive interpretation just by looking at the clause words led me to think that INITIALLY DEFERRED would not check record validity when a constraint is added to a table, but obviously that's wrong too.)