Different AMI based on AWS Region?

Is there any reasons that AWS AMI are different based on the region?
For example, if I try to launch an EC2 instance in Frankfurt I Got this AMI:

Amazon Linux AMI 2015.03 (HVM), SSD Volume Type - *ami-a8221fb5*

While in Oregon:

Amazon Linux AMI 2015.03 (HVM), SSD Volume Type - *ami-e7527ed7*

Note the different ami-version: ami-a8221fb5 and ami-e7527ed7.
Is there any reasons?

And how can I build a cloudformation template that I can launch on both Frankfurt or Oregon without having to change the ami ?