Repeatable read: the transaction may not be serializable with respect to other transactions

The text below is from ‘Database System Concepts, Silberschatz. I can’t understand the bolded part. How the transaction may not be serializable with respect to other transactions? And also how a transaction may find some of the data inserted by a committed transaction, but may not find other data inserted by the same transaction?

Repeatable read allows only committed data to be read and further requires that, between two reads of a data item by a transaction, no other transaction is allowed to update it. However, the transaction may not be serializable with respect to other transactions. For instance, when it is searching for data satisfying some conditions, a transaction may find some of the data inserted by a committed transaction, but may not find other data inserted by the same transaction.