Cleric Channel Energy heals and damages at the same time?

Can a Cleric use Channel Positive Energy to damage and heal at the same time?

For example: An undead and a living human are 10 feet from the cleric. The cleric Channels Positive Energy. Does he damage the undead and heal the human?

If he can only heal or cause damage, and not both at the same time, why not? There is not much sense behind this. If it is a burst of Positive Energy shouldn’t it affect all in the area in some way? How does Positive Energy stops healing living creatures or stops causing damage to undead?