A lil confused here from BOL

From BOL : How Transactional Replication Works
"After all Subscribers have received transactions, the Distribution Cleanup
Agent removes delivered transactions in the distribution database. Delivered
transactions are kept in the distribution database for a defined period
known as the retention period. Setting a retention period while scheduling
backups can ensure that information required to recover a destination
database automatically is available within the distribution database.
For example, if a Subscriber has scheduled a transaction log dump of a
destination database every 24 hours, you could set the retention period to
48 hours. Even if the Subscriber experiences a failure immediately before a
scheduled backup, all transactions necessary to restore the replicated
tables automatically will still be available to the distribution process of
the Distributor."
So im confused at the first 2 statements where it says that the cleanup
agent job removes the delivered transaction in the distrib database but then
it also states that those delivered transactions are kept in the distrib
database for a defined period and then gives an example that we could set
the retention period to 48 hrs.. So if the cleanup agent cleans them, how is
it retained ?
Hassan,
they're retained until all subscribers have subscribed or the retention
period is reached, whichever is shorter. In the case of anonymous
subscribers this changes and they're retained until the end of the retention
period even if subscribers have picked them up.
Rgds,
Paul Ibison SQL Server MVP, www.replicationanswers.com
(recommended sql server 2000 replication book:
http://www.nwsu.com/0974973602p.html)