Per the help text in the survey configuration:

"Default Source Code:
Specify the default source code for this survey. The code is added to the Profiles of constituents who complete this survey and who are not already identified by another source code."

What this means is that if the constituent already has a source code stored in their session from some other part of your site, that source code will trump the default source code entered into the survey. To get around this, the source code would have to be built into the URL - For example:

Where "survey1111" is the source code.