I've had things like this happen more than once: 1. HR/recruiter-ish Phone screening 2. Engineer-ish phone screening, sometimes with a coding session 3. Another phonecall as a prep for further rounds of interviews 4. Travel that requires me to take at least one day off of work and travel to another city for an in-person interview 5. Rejection with no reason, and "We have a policy of not providing that information to candidates" if you ask why 6. Questionnaire from the company, asking for feedback on their interview process (snerk)

In your case, with one guess, I'd say that the test cases that were provided were just examples of things you'd have to do to pass. It's like the homework assignment where the professor gives you a problem to solve and some test inputs. You know that they'll run some "acid test" on your code after you've submitted it. Maybe it caught some corner cases that they considered essential.

That, or they didn't like something about your coding style (style is subjective, and you'll never have feedback on how they judged it). Or, or, or...you can come up with speculative lists all day.