Pg isolation levels: 3 or 2?

Hi all.

From the current docs (Ch.13 on Concurreny Control):

"In PostgreSQL, you can request any of the four standard transaction
isolation levels. But internally, there are only three distinct isolation
levels, which correspond to the levels Read Committed, Repeatable Read, and
Serializable. When you select the level Read Uncommitted you really get
Read Committed, and phantom reads are not possible in the
PostgreSQLimplementation of Repeatable Read, so the actual isolation
level might be
stricter than what you select."

How is that three levels and not two? Read Uncommitted and Read Commited
are the same. And Repeatable Reads don't allow phantom reads thus making
them effectively the same as Serializable. No?

TIA,
Thalis K.