Inconsistence in transaction isolation docs

/From:
http://www.postgresql.org/docs/8.2/interactive/transaction-iso.html

"
Read Committed/ is the default isolation level in PostgreSQL. When a
transaction runs on this isolation level, a SELECT query sees only data
committed before the query began; it never sees either uncommitted data
or changes committed during query execution by concurrent transactions.
(However, the SELECT does see the effects of previous updates executed
within its own transaction, even though they are not yet committed.) In
effect, a SELECT query sees a snapshot of the database as of the instant
that that query begins to run. Notice that two successive SELECT
commands can see different data, even though they are within a single
transaction, if other transactions commit changes during execution of
the first SELECT.
"

to me the above sentence sounds inconsistent: it's asserting that both
1) and 2) apply:

1) it never sees ... changes committed during query execution by
concurrent transactions

2) Notice that two successive SELECT commands can see different data,
even though they
are within a single transaction, if other transactions commit changes
during execution
of the first SELECT

Can anyone explain, please?