timestamp format in 12c

We have a query below:

select to_char(EVENT_TIMESTAMP,'MM/DD/YYYY HH24:MI:SS.FFFFFF') Timestamp from unified_audit_trail;

DATE
 --------------------------------------
 01/25/2015 10:19:29.725385325345625375
 02/27/2015 11:17:17.034196027191037156

My question is what was got is not what was wanted.  I expected 01/25/2015 10:19:29.725385 for 1st record, but it gave me 01/25/2015 10:19:29.725385325345625375.
1. How can I get what I want?
2. Does the many digits after decimal point mean a high precision of fraction of a second?

Please can any gurus shed some light on it?