Let's admit it (your username does), you're very good. Code Jam seems to reward problem solving and maths/CS ability far more than programming ability, relative to other coding competitions. The most important thing is analysing the problem, not memorising standard algorithms. That's why you could do so well with so little competition experience. And why I have stuffed up so many GCJ rounds.
That said, sometimes the field really is weak. I finished in the top 1000 on the first Distributed Code Jam and got a t-shirt just because... less than 1000 qualifying contestants were willing to even figure out the novel setup and compete. (Actually I did badly.)
Most people will not be able to solve longest common substring without being familiar with the problem ahead of time, and many programming problems can be broken down or transformed into similar algorithms.
Identifying root applicable algorithms and reproducing them quickly is what you study when competing in programming competitions.
That said, sometimes the field really is weak. I finished in the top 1000 on the first Distributed Code Jam and got a t-shirt just because... less than 1000 qualifying contestants were willing to even figure out the novel setup and compete. (Actually I did badly.)