Strange microseconds issue when querying by time

gistfile1.rb

# Please have a look, how updated_at timestamp is represented in generated SQL

# In this example everything is OK
Event.where(Sequel.expr(:updated_at) > Time.at(1405338941.821000)).sql
# => "SELECT * FROM \"events\" WHERE (\"updated_at\" > '2014-07-14 13:55:41.821000+0200')"

# In this example, there's a precision issue (see microseconds), why?
Event.where(Sequel.expr(:updated_at) > Time.at(1405341161.918000)).sql
# => "SELECT * FROM \"events\" WHERE (\"updated_at\" > '2014-07-14 14:32:41.917999+0200')"


# So lets try pure parsing, but everything seems to be ok.
# Am I doing something wrong?

"%10.6f" % Time.at(1405341161.918000).to_f
# => "1405341161.918000"
"%10.6f" % Time.at(1405338941.821000).to_f
# => "1405338941.821000"