An instance brought up in a VPC with internet access is able to respond to traffic directed to an internet routable public IP assigned to it, as well as the internal subnet IP it has on it's only interface.
How is that accomplished?
edit for further information: This is an standard EC2 instanced created inside a VPC I have made that does not have a NAT instance.
The box has an public Elastic IP applied to it and can send and receive over this route, despite only having one interface with a private 10.x address.