Timezone offset displayed though it shouldn't be stored
From the PostgreSQL documentation you link to, about halfway down the page:


  When a timestamp with time zone value is output, it is always converted from UTC to the current timezone zone, and displayed as local time in that zone. To see the time in another time zone, either change timezone or use the AT TIME ZONE construct.


So (as Evan Carroll hinted in his comment), if your system has 'Europe/Berlin' set as timezone, you would expect to see the UTC date/times output using that timezone.

To confirm, try inserting a date/time from another timezone; that should also be output showing the 'Europe/Berlin' time instead. Or, try changing timezone, and confirm that the dates that were showing the time from Europe/Berlin now show the time from the new time zone instead.