It was only after I was later hired into Google that I learned from my original recruiter that I had actually done quite well in my first interview process. What I did not know at the time of my rejection letter was that they had narrowed down their pool of potential applicants to about 1200 resumes for three open positions they were looking to fill. Apparently, I made it into the last round of 10-12 applicants and just did not have the experience level with the specific tools for the job as others did. Thus, I received the same rejection letter the other 1196 people received, and never even knew I did as well as I did.
My advice would be to just stay in contact and keep Google updated anytime you have something new added to your resume or skill set. Based on what you have described, it actually sounds like you might have done pretty well. There are too many factors going on behind the scenes to say one way or another why you did not make it through this time.