DBMS LOCK.ALLOCATE

Dear All,i would like to write a question.
The Table "dbms_lock_allocation" is used by  the package "dbms_lock", isnot it?
In case of yes:
The expirationtime is 10 days.
But is i watch at this table all values are very flexible.

This SQL-Statement dellivered sometime "o" sometimes 4400
 select count(*) from dbms_lock_allocated where expiration < sysdate;

and this SQL-Statement: sometimes 1200 sometime 44201
select count(*) from dbms_lock_allocated;

Who can explain me which process and  which circumstandes are responsible for this work.

Are modifications allowed?

Thanks for any answers.
Kurt