Actually, the moon is also playing a role here. The reason we gain weight in the PM is the same reason we get a high tide, because the moon is exerting a stronger pull on our little section of the planet. If you don't experience at least a SLIGHT weight gain after the sun starts setting, it is probably due to hugely fluctuating water-levels in your body.... normal for people losing aeight, but also a factor we must assimilate into our weigh-ins.
And of course, we usually have 'lost' that weight once we weigh ourselves in the morning. Another reason not to do daily or multiple-times daily weigh-ins.... too many factors that come into play.