Timestamp displays different with and without TO_CHAR

[Worker]

Can someone explain why PL/SQL Developer shows two different results from this query? The only difference is the explicit to_char. I don't know how PL/SQL Developer does the implicit to-character conversion to display the first value. Does this have something to do with time zone rules on the server being different (more up-to-date) than those on the client?

select         from_tz(cast((TIMESTAMP '2013-03-27 10:00:00 US/Eastern') as timestamp), 'US/Eastern'),
       to_char(from_tz(cast((TIMESTAMP '2013-03-27 10:00:00 US/Eastern') as timestamp), 'US/Eastern'), 'DD-MON-YY HH.MI.SS.FF6 AM TZR')
from dual;

result:

27-MAR-13 09.00.00.000000 PM US/EASTERN
27-MAR-13 10.00.00.000000 PM US/EASTERN