A website which is mobile compatible usually achieves this in one of two ways:
a) The website detects the device viewing it and displays an alternative theme which is designed to be more user friendly with the smaller screen size.
b) The website does not care what the device is. It takes into account the browser window size. The content repositions itself to contantly fit within the window regardless of the device.
The downfall of a) is that there are so many different devices out there, it is difficult for a website to be configured to recognise them all. The website may produce mobile compatible content if viewed on the common iPhone, but what about the many other devices that are not mainstream? Every time a new device comes out on the market, can you trust that the site owner is going to arrange for the site to be updated to recognise that new device?
This is why b) is a better option. It will just work all the time. That is why we use this method.
We also recognise that mobile compatibility should not be a premium option that costs extra, and that is why is it just part of what you get with a website by Mills Web Design. In this day and age, mobile compatibility is just expected.