How come Datagram Packets between same source-destination pair may take different routes? This might be a rookie question, but I wanted to understand the following line.

My book says, each time a host wants to send a packet, it places the address of the destination host in the packet's header. Okay, I understand this. 

Also, packets forwarded using destination address have their address mapped to an outgoing interface. Packets between same source destination pair may take different routes. 

Why is that? If packets have the same destination address should they not be mapped to the same outgoing interface? 

Is there something I have left out or not understood that is leading to this confusion or is there an explanation to this!
Packets are routed individually: each time a router receives a packet, it decides what to do with that packet. It doesn't necessarily make the same decision each time it receives packets for a particular destination. To give a trivial example, somebody might have changed the router's configuration in the time between two packets arriving; a more common example would be a router trying to avoid congested links by sending some packets down a quieter link.