One of the main factors that may still be linking your real name to your new user page (new since the username slug is a part of the URL) could be Google's automatic synonym searching.
As Bobby pointed out, the cache believes "these terms only appear in links pointing to this page". However, I've tested that out before and I don't believe that's the whole story. It actually seems to be Google's canned message for any terms that were only indirectly related to the results page, including but not limited to links pointing to the page.
As seen from this announcement on the Google blog, Google has started executing searches based on related words automatically:
Understanding queries: It is critical that we understand what our users are looking for (beyond just the few words in their query). We have made several notable advances in this area including a best-in-class spelling suggestion system, an advanced synonyms system, and a very strong concept analysis system.
This synonym searching is both dynamic and contextual, in that it learns as it crawls, and relates terms based on the context they are found in.
So the fact that you recently changed your username probably means that Google has a fairly good guess that Your Real Name is strongly associated with sgusc or with your ID (in the URL, which didn't change) or possibly even multiple data points on the page (many links which have only ever appeared to Google on your page — your specific questions and answers, for example, since each link includes the ID of the post you actually made, not just the whole question ID).