I passed all the test cases when running my code. Why didn't I score 100%?
On each run, the test suite runs a random subset from a larger group of tests. It's possible to pass all of the randomly selected tests during a test run in the IDE, but then fail different randomly selected tests on submission.
It's also possible that while your solution is logically correct, it's inefficient enough to run into the execution time limit allotted for that test.