Directory Browsing in ASP.Net

Friends,

I have one real time scenario here, in my asp.net web application i gave directory browsing = true and i can able to browse the directory files from the root folder in the IE browser. But when i tried to load the Web.Config file alone it gives error. What is the possible idea behind this, i am sure opening webconfig from IE is security issue since it has all the connection strings and credentials. How exactly IE detects not to open the WebConfig alone??

Any suggestions and Comments???