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 PostgreSQL implementation 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?