See the query and results I show in the screenshot below. It's showing a time zone for the TIMESTAMP WITH TIMEZONE column, EFFECTIVE_TIME because I changed the setting in DbViz to do show them. Note that the results all show EDT.
I then used the to_char function so that Oracle determines the timezone, not DbViz, and Oracle returns what the real timezone is for each:
You can see clearly that not all the times are EDT. Some are EDT, some are EST. So, DbViz is not displaying the timezones correctly on the first query above. It's as if DbViz is checking that 10/20/2017 is before the end of Eastern Daylight Time (which ended on 11/5/2017) and so displaying them as EDT.