Lock table, Select for update and Serialization error

Hi,

As per postgres docs, 'Select for update' is used to obtain row level
locks where as 'lock table' is used to obtain table level locks.

Under serializable isolation level, select for update gives error if
rows selected have been modified concurrently.
but 'lock table' does not give such error even though some of the rows
in the table are modified by concurrent transaction.

Is this the expected behavior?