I am not 100% sure about this, but this is the normal way of consideration. Normally, Google also looks at which data center the user has been querying and based on that it decided the location. It tries its best to give you the location accurately, but sometimes, the mechanism fails, therefore the not set comes in.
Managing a Large E-commerce Website
9/20/2012 How to start managing a large e-commerce website: from infrastructure and seasonal products to working with an awesome team at the enterprise-level.