Pasta cooking time vs. box instructions
One very likely explanation: did you start your pasta in cold water? These times are given for pasta that gets immersed in a very large amount of already-boiling water. If you either started from cold, or had a rather crowded pot of pasta (such that the water cooled down when the pasta was added), then the time needed will increase. 

For me personally, using the boiling water method, the times on the pasta box have always been correct.