python unix timestamp not match with different timezone

Question

when print the following timestamp utc Jan 30 2021, I am expecting they are the same. why there is a 340 difference?

start = datetime(2021, month=1, day=30, hour=21, minute=0, second=0, tzinfo=pytz.utc)
start1 = datetime(2021, month=1, day=30, hour=16, minute=0, second=0, tzinfo=pytz.timezone('US/Eastern'))
print(start.timestamp())
print(start1.timestamp())

output:

1612040400.0

1612040160.0