A friend from my SAR team forwarded Michael Coyle post on GPS Accuracy. Don't get too wrapped up with the complexity but take away the basis of your receiver's accuracy calculations.
Your GPS is lying to you
At the very least, you’re probably misinterpreting what it’s saying.People’s attitudes toward GPS devices are very interesting. Since 2000, when selective availability was turned off, GPS has become ubiquitous to the point where most people have one and interact with it on a daily basis, whether they know it or not. To make things even more confusing, the general case of global positioning, whether satellite based or not, is completely pervasive. You may not know this, but this web site knows where you are (to some level of detail) right now.
To read the rest of Michael's post go here.