Why the extra minute in PostgreSQL when using time zone info?

I am creating some test data for a DateTimeField using the following function:

    def rndDtTm(self):

        uktz = timezone('Europe/London')

        year = 2015

        month = 6

        day = randint(1, 30)

        hour = randint(0, 23)

        minute = 30 * randint(0, 1)

        return datetime(year, month, day, hour, minute, 0, 0, tzinfo = uktz)

When I look at the PostgreSQL field in pgAdminIII the times are either 1 or 31 minutes past the hour:

2015-06-10 22:01:00+01

2015-06-12 21:31:00+01

I was expecting them to be 0 or 30 minutes past the hour.  Why the extra minute?

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