Dichotomy defies definition

Mercury and Venus are approaching their greatest elongations--Mercury in the morning and Venus in the evening. That's when they're farthest from the sun in the sky and can be seen most easily. The two planets are also predicted to be at dichotomy around June 4. But an unexplained phenomenon is likely to make one appear to be in its semi-circle phase around the middle of this week and the other around the middle of next week. Why do the appearances of Mercury and Venus defy geometry?

Mars marches into the Beehive Cluster. Get a telescope or binoculars for a splendid view.